Backyard Aquaponics
A place to discuss aquaponics

Hi All,

I need help with silver perch deaths.

I have a large aquaponic set up in my backyard swimming pool.
At normal water level it holds 65,000 litres.
System has been established and evolved over about 4 years.
Currently has 11 grow beds, running from 3 different pumps, each going through a radial flow filter.
Pool currently has:
approx 100 perch in 2.5 to 3.5 year age group - free ranging in pool
200 at 1.5 year age group - in pool in holding net cage set up (see pics)
200 just over 6 months old - in pool in holding net cage set up (see pics)
150 trout 4 or 5 months old - in pool in holding net cage set up (see pics)
Pool also has lots of tiny native fish, snails, 2 x 1m2 water treatment floating rafts.

Water tests as of this morning - levels have been similar for a few months

pH - 6.2 (I had 6 back in May, but has been slowly climbing, and is usually 6.4 lately)
Ammonia - 0.25 to 0.5
Nitrites - 0
Nitrates - approx 80 to 100
KH - 3
GH - 5
water temp - 13

Have had silver perch dying over the past 10 days. Approx 30 fish have died in that period. Mostly they are the youngest ones and the smallest of that group, however there have been a few medium size fish too. Looking at the DPI manual for fish health I think they have a fungal disease (Saprolegniosis).
Please see photos. If you have any experience with this disease I would love your input.

I salted my pool to 2PPT (parts per tonne - or kilos per 1000 litres) last week and added another 0.5PPT yesterday.
Am thinking about adding more - DPI recommends 2-5PPT for Saprolegniosis.

Also I am wondering about my water quality issues. Obviously I would love to get my ammonia down to 0, my nitrates lower and my pH higher. I am wondering if anyone can shine some light on the relationship between carbonate hardness (or lack of) and pH and whether this influences nitrates and ammonia.

Or any other ideas about anything would be welcome!
Thanks heaps in advance.

Sorry to hear about the fish deaths. The ammonia shouldn't concern you too much with the water temp and pH as low as they are.

I think the volume of filtration for the number of fish is probably the main contributing factor in your water quality but I'm not certain. What is the total volume of the grow bed filtration area? Any idea of the total Kg of fish in the system?

With low carbonate hardness you can get wider fluctuations in pH. Usually adding carbonates will raise the pH but sometimes other buffer systems are at work. 6.2 is not a bad pH for AP. If you do adjust the pH up then you'll need to be careful because this could cause some of the ammonia to convert to the more toxic form.

Those shade cloth covers Shade cloth normally has mould inhibiters it may be a possability

What total weight of feed are you giving the fish each day? In addition to the fungal problem, I suspect you need more GBs.

Thank for the replies everyone.

Some answers to your questions:
I estimate the total weight of fish in my system to be about 110kg
That being made up of 100kg of perch and around 10kg of trout

Currently we are feeding a total of 350grams food per day (trout fed twice and perch fed once)
Estimate that it is 200gm of that to the trout and 150gm to the perch.

This is unusual at the moment due to our situation.......
- Generally we have reduced feed a bit over the past week, in case issues coming from water quality.
- One floating cage (with approx 160 x 8 month old fish) has not fed for past 10 days or so (that is the one where the most sick fish are coming from, so guess they are all a bit under the weather).
- We only feed the fish that free range in the pool (the 2.5 to 3.5 year old fish - total weight about 40kg) once every week or two in winter as generally they don't feed much in winter and have plenty of aquatic snails, azola, sweet potato leaf and natural food in the pool.

However that being said, I do generally underfeed on pellets according to the tables as there is a lot of natural food in the pool and then I throw some maggots and worms to them sometimes too. (Eventually would like to get off pellets altogether, but thats another story). Up to this point fish have been happy and healthy.

In case its useful - I feed the perch Skrettings Nova and the trout Skrettings Spectra

Our total grow bed volume is 11m3 - adding an additional 2m3 next week!

Other factors, in case they are useful.....I use radial flow filters on the 3 pumps hooked into the pool. They are cleaned each week and we put the solids into a mineralisation tank (as per Murray Hallam's system) with comfrey, seaweed, compost and mollasses and heavily aerate it. We then tip approx 8 to 10 litres of the settled component of this mix on the grow beds each week.

Food&Fish - what did you mean about the mould inhibitor on the shade cloth? Do you think it is bad for fish health?

Thanks again.

Hubby just told me I have to discount for stones in total bed volume, but not sure how to do that!

Shade cloth comes with some sort of release agent on it- it's always a good idea to wash that off as much as possible before using it around fish.

That's very light feeding- I have 430 trout in my tank and they eat up to about 1kg per day. I have 6m^3 of GB media, and will be adding another 2.5m^3 to that soon, plus a swirl filter removes much of the larger solids, which go to a separate system with WBs.

You don't discount for stones in the GBs, you count the total wet media volume, ie the wet rocks.

I'd make a really really thorough check for any dead fish that you may have missed - especially in the cage with the most deaths. Look for any areas of decaying organic matter in the tank and eliminate them

I seem to be suggesting UV Filtration a lot lately and this isn't going to be an exception - It could cut down on the transmission of any pathogens that are free floating in the water and might help you get control of this. It will also take out the free floating algae.

Thanks All for your comments.

I had a visit today from the Fish Vet who did gill inspections on both dead and sick fish and put some slides under the microscope. She has diagnosed Ich (white spot) with a secondary fungal infection of saprolegniosis.

According to her tests, water quality is fine, although she recommends increasing my carbonate hardness.

Her recommended treatment for the Ich is formalin I really don't want to do this. My water is already salted to 3g/litre. Anything else anyone can recommend?

I am thinking to try and avoid this by harvesting all my edible size fish and loading up friends and family's freezers. Then I will take my chances with the smaller fish. Is this feasible??

Scotty435 - I am interested in your idea about UV filtration. As I understand from the vet, even if I get all the fish out who are infected with Ich (impossible as there are too many hiding places in my pool) the Ich protazoa will still be circulating in the pool. Does UV filtration remove protazoa?

Also Scotty435, can you explain what is the problem with free floating algae?

If you have white spot my advice is get them in the freezer
I had a outbreak in one system and tried to treat them ended up losing most and just killed the rest in despair
Lost 80 big fillets
The more you try to treat them the more stressed they get and it spirals out of control

Symptoms
White spots like grains of salt on head
Normally the fish would huddle in a tight school when I looked in the tank
Once infected they cruised around sort of half blind and wouldn't feed

I took all the fish out Nuked system with chlorine , drained system and I am leaving it empty until spring
Apparently in cold water it can survive for 6 weeks or more without a host (fish)

I have another system 2 steps away that has no sign of disease it has me stumped where it came from.
I havnt lost fish to disease before

Read about the life cycle in cold water they multiply rapidly
Best way to knock it off is to run your water pretty hot for 4 plus days , not really practical this time of year and in your big pool more or less impossible.

If you have Saprolegnia as well you have your hands full
My mate lost a heap of fish to that one last yr when the water got real cold after a run of minus temps overnight.
I would certainly try more salt

A UV sterilizer can kill some stages of the ich parasite (the free living ones (not the ones attached to surfaces)). Basically I'm saying it would help and I'd be running mine if my system had ich because I think the combination of UV and salting is better than just salting.

If I were you, I'd give it some time with the salt concentration kept at 3 parts per thousand - this has worked for others here but it does take time especially at lower temps because the life cycle of this parasite takes longer. I can't say exactly how long but I'm guessing the life cycle will be around 15 to 30 days at this temp.

I wouldn't add Formalin in my AP system. If nothing else you should know that it will likely have an affect on the bacteria acting as your biofilter. While this isn't directly analogous to our freshwater systems, here's an example from marine aquaculture - https://www.cambridge.org/core/services/aop-cambridge-core/content/view/F16657527317931E7E0EA0EBDD0C6A4F/S0990744002011981a.pdf/div-class-title-effects-of-formalin-chemotherapeutic-treatments-on-biofilter-efficiency-in-a-marine-recirculating-fish-farming-system-div.pdf. Formalin is a combination of Formaldehyde and methanol. Formaldehyde is a known human carcinogen but it is also found normally in the human body - so the amounts used are important. It's hazardous to handle in concentrated form.

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I don't mind seeing some algae along the tank walls and even some algae suspended in the water. Algae is really kind of a mixed bag but when you have a lot of algae it's taking up food that your plants and the bacteria in your grow beds would otherwise be getting. Algae also can cause large fluctuations in the pH of the water over the course of the day when your carbonate hardness is low and this can stress the fish. Algae sometimes will die off in large quantities due to existing conditions and this can release ammonia or use up oxygen as it is broken down (this can also be a concern when using a UV clarifier to remove algae). On the other hand some fish, like Tilapia can actually use the algae as food and it does provide food for other organisms that fish might feed on. Most AP systems here go through a pea soup algae stage which eventually passes and then they typically don't have anymore problems with it unless the system gets too far out of balance. Based on your nitrate readings you probably have a slight overabundance of nutrients at the moment.

Salting to 3ppt has removed Ich from infected fish I have purchased, along with a UV steriliser for the 2nd incident. I lost no fish at all to Ich, out of hundreds that were infected.

Photos of Ich on my 1st batch of rainbow trout here: viewtopic.php?f=18&t=16345&start=105#p398279


Don't use formalin in your AP system!

Hi All,

Just a quick update on my fish for those who are interested.

For last two weeks we have just had 3 deaths and last 10 days no deaths at all

The strategy we used was as followed:
We harvested 60 large perch which took a small load of the system.
We salted the whole pool to 3g/litre and to take any sick looking fish and put them in a hospital tank at 5g/litre.
We used vitamin C - at 10g/kL and added cod liver oil to the fish feed. The vet also suggested crushed garlic juice to the feed too, but we haven't done that as yet (guess we were holding on to an additional therapy in case needed).

Feeling like things are good now. I think what happened for us was that the smallest of our young batch of fish got hit hardest and the weakest ones died, the others rallied under the salt water treatment, vit C and cod liver oil and now all seem a bit healthier....or maybe just survival of the fittest!

Thanks again for everyone input!

Don't forget outside elements. Sometimes neighbours have their homes sprayed for Whiteants and spiders. That spray travels in the breeze and is so toxic to fish. If you have a sudden spate of death ask neighbours if they have sprayed and to inform when they plan to spray in the future so you can cover your growbeds and tanks

I think that did happen to us last year.