Backyard Aquaponics
A place to discuss aquaponics

I have system with about 50 mature silver perch (20-30cm). Since about September last year I have had the water green with algae and it will not go. I have had algae attacks like this in the past but usually clears after a couple of weeks. My fish are dying at a rate of 1 - 2 a week. A couple of months ago I tried a complete water change, but the algae returned in days.

Value

Any chance of pics of the system or description and water parameters? Bit hard to go on without any detail.
As an interim you could try pumping the water through an external filter, dosn't have to be sophisticated, Half a blue barrel with gravel foam then that white pillow filling stuff (?dacron) on top. Will get clogged quickly so have to keep changing the top layer. Problem is if you kill it without getting it out could lead to other problems.

+1
Some idea of the system might help...
-volume of water and GBs
-How much you're feeding

Do you have a cover for the FT?

I would think green water is a result of too much nutrient from over feeding or not enough GB volume combined with too much sunlight.

I'd suggest covering the FT with shade cloth and reduce the feeding.

My 2c worth
Derek

I suspect the pH might be wacky as well... which is probably inhibiting the bacterial filtration...

Full set of water test results needed please... including water temp...

I 've attached a pic of my system. It is 1700 litres pond and 800 litres of grow beds. the ph of the water is 6.2. I feed about 200 ml(10 tablespoons) of pellets every day.
I'm not convinced filtration is the answer. As I said I did a complete water change but the algae returned.

Plenty of growth...
But that doesn't always mean a lot.
Do you have a way of working out how much flow you have?
Do the delivery lines to the GBs come apart?
I think I would be checking the lines for blockages that reduce flow.

"I'm not convinced filtration is the answer".
Said "as an interim measure to clean it up" . IMHO Would certainly cover some of the FT also the drop from the gb's would be better out of the water to provide more aeration. The ph is probably all over the place, with that much algae, 6.2 stable you could get away with prefer slightly higher myself. What C1 said re slow flow, got to have a lot of roots
Would still go the filter, as an interim measure, cut back the feed a bit and check everything else. Big Pipe rainwater?

I am.... you're feeding about 100gm/day (as you should be for that amount/size of fish)....into a 1700L with only 800L of growbed.... a ratio of 1:0.5.... not even 1:1

I'm assuming that the 800L of growbed is the ACTUAL volume of the growbeds.... not the actual volume of the media.... i.e ... your FILTRATION volume.... which I suspect is probably only about 400L

People ask about the 2:1 ratio... particularly as many systems seem to work OK with a 1:1 ratio....

But it's all about the filtration capacity.... and the ratio is based on actual growbed volume...

When you consider the amount of real filtration volume (typically about 50%) of a growbed...

Then even in a 2:1 ratio system.... the filtration capacity is actually only equal to the FT volume....

This is why it is considered appropriate for a "mature" system... and/or a system with a higher stocking density.... or higher BIOMASS of mature fish...

Your system has a "high" biomass of mature fish producing a high nutrient load.... ammonia and a "high" nutrient load from uneaten feed and fish excretions....

With only a filtration capacity of at the minimum 1/2 of what you really need.....

Your growbeds just can't process the nutrient load fast enough IMO.... and coupled with sunlight access to your tank.......ALGAE.....

So yep, changing your wate helped for a day or two.... but all the conditions are there for it to just rapidly bloom back...

Your saving grace at the moment is your pH... at least for plant growth.... if it gets to 6.0 or beyond .... then it could begin to compromise the bacterial colony...

I repeat the suggestions..... shade the tank, to basically exclude as much light as possible... cut back the feed for a few days, until you've got the algae under control... and buffer your system with some shellgrit to move the pH toward 7.0....

I'd also seriously suggest that you double your growbed capacity...

Oh... and get an airpump and at least one (preferably two) airstones going in the fish tank .... immediately....

With warm water temps... and algae.... you'll be compromising your DO levels... particularly overnight....

rupe your numbers are generally quoted for 3kg/100L stocking density...this system is running not much more than 1kg/100L at a guess.
My advice is the same though:
get a test kit and measure ammonia and nitrates at least
if nitrates are high then plant growth is not keeping up
if there is any ammonia then bacteria is not keeping up

either way, feed less, shade the tank.

50 Fish in 1500L (+ a bit).... is 3kg/100L by my reckoning Steem... if they grow out to 1kg each....

Even at >500gms each... it's the size of the fish, and the feed load to be processed... the total nutrient load... that I'm considering... and IMO... it still needs about double the capacity the system currently has...

I'm not suggesting he needs to go to a 2:1 ratio.... but certainly to a 1:1.... these aren't fingerlings eating dust...

Remember at full size (mature) we suggest the 2:1 ratio.... for that reason .... filtration...

He's halfway there... so needs at least 1:1 IMO...

and looking at the algal line it would appear that the FT is not full.
Looks like it could do with another few hundred litres

You can also try barley straw pellets to rid the algae problem. It's mainly used in the pond world. Barley straw itself takes 4 months to ferment but the pellets or extract will start working immediately. Aside from that, each time you change the water, you are introducing new water that algae hasn't attacked yet so it will.