Backyard Aquaponics
A place to discuss aquaponics

My 500L ST has accumulated a fair bit of gunk on the bottom, and some of the floating fish pellets are also in there with that bacteria infested "fuzz" around them (surprising, given they have to sink to get sucked up into the overflow, yet are floating in the sumo?).

Obviously, my pump in the sump isn't doing a great job at keeping it clean. I'm concerned about simply stirring this stuff up, allowing/hoping the pump to suck it up and disperse it throughout the GBs.

Any ideas?

I'd use a skimmer net to scoop out any floating yucky stuff and put it in the compost.

Then while the pump is running you might be able to fan the gunk on the bottom into the pump to go up to the grow beds. If there is too much gunk, perhaps just skim it out into the compost.

This is the reason that having the water go directly from the fish tank to the grow beds is such a benefit.

Perhaps if you can have the inflow into the sump go in at an angle, it might help create a swirl effect that might help the pump suck up more of the solids. Otherwise, if the sump never drains too low, you might be able to keep a few fish in there to help stir up the gunk and make it easier for the pump to suck it up.

Make sure to check the pump and make sure it isn't partially clogged up as that can greatly reduce it's ability to clear the solids.

Couple of things Ryan...

Concerned that for some reason you have "fish pellets are in there with that bacteria infested "fuzz" around them ".... scoop them out immdeiately

I'm assuming that your fish are in a seperate tank that supplies your growbeds...

The question then is why/how are the pellets making their way to the sump???

It implies two things....

One.... you are grossly over-feeding....

Two your growbed filtration either doesn't have sufficient capacity to process the nutrient load.... and/or is doing so in-effectively... perhaps due to design...

I'm somewhat at a loss to understand just how the pellets can possibly get back to your sump??? .... I would have thought that they would remain trapped in your growbeds...

Regardless... I would suggest you do two things....

Seriously reduce your feed.... and then clean and dump your sump tank....

Look at your growbed irrigation and drainage design to determine why/how any pelets might be returning to the sump.... and rectify.....

Reducing your feed may solve much of the problem.... but if you are required to feed the amount you are, or will be in the future...

Then it means you will have to increase your filtration capacity... i.e. add more growbeds...

My concern is whether or not your current growbeds may actually be clogged with unprocessed feed and other solids.... and about to turn into ananaerobic festering mess....

Have you got any worms in your growbeds??

After reading TCL's post above... I realised that you are pumping to your growbeds from your sump... and assume that the pellets are getting there from an "overflow" in the fish tank...

In which case you are obviously overfeeding... and it should be apparant ....

Cut back the feed... immediately... do as TCL suggests... and consider the other points I raised...

Thanks for the advice. I should have mentioned that there wasn't heaps of pellets in the sump, about half a dozen or so. I'm baffled how they could sink to the bottom of the FT, where the overflow sucks from, then float again in the sump?

I've just changed up from 1mm to 4mm pellets, which the fish have taken a couple of days to get used to. They now seem to be eating all I give them within a couple of minutes.

Most of the gunk in the ST is on the bottom. I'll scoop out as much as I can, and try and steer the rest into the pump. The ST has a lid on it, with about a 1 inch gap where the irrigation pipe exits. Today was the first time in a couple of months since I've opened the lid and inspected it. I was thinking of adding crayfish. Would they help remove this gunk, and would they survive ok with the lid as it is?

Not sure but most of the pellets sink after a while. And if they start to rot they would float again.

Ahh... OK.... qiute a big jump from 1mm to 4mm pellets...

They'd certainly take a day or two to get used to them... and yep... the bigger pellets do tend to "float" more... even the "sinking" pellets.... until they soften and sink...often overnight...

So they could quite easily get sucked up the SLO pipe... then they'll float as they start to rot as Duff says....

Even some my 3mm pellets make it into my growbeds...

If it's only a few... no dramas... just scoop them out...

any chance of getting some redclaw in there ryangus?
I am sure they would make short work of it

Perhaps you could work out a way to have a small grow bed between your fish tank and your sump, (even if it is too dark there for plants, it could act as a filter) to keep some of the gunk out of the sump tank? Perhaps even set it up as a 5 gallon bucket with some holes in the bottom as a flow through (still put a bit of a overflow on it though) type filter and when it gets too gunky you could take and rinse it over some dirt garden plants? Just an idea.

I think there is a correlation between fish growth. vegitative growth and bio filtration.
The trick (IMHO) is to balance the fish feeding with the growth rates of plants etc.

eg trying to grow too many plants with a small fish load does not provide enough nirate
so the plants suffer...

Not enough plants and lots of feeding doesn't allow enough nirate 'take up', result is too much
buildup of bio matter. (just how I see it)

How many fish are in the tank, what size and type?

What fish stocking rate do folks recommend to suit a 500lt STs / FT to make it viable?

We generally recommend 3kg/100L as a safe stocking rate...

That's 3kg of fish biomass per 100L of water... and assumes at least a 1:1 growbed:fish tank volume...

I prefer to think of it in terms of filtration capacity.... i.e. 3kg of fish per 100L of growbed filtration capacity....

IMO... this allows for grow out of fish to approach 1kg each.... even if your growbed capacity doesn't increase to the 2:1 ratio suggested for a mature system with fish at edible stage...

So... if you're intending to eat your fish at around the 500gm mark... then in 500L... about 30 fish is nice and safe...

Of course if you increase your filtration capacity... with additional growbeds or external filtration devices, solids removers etc... then you could stock higher....

But 500L tanks are small... prone to quick temperature changes, pH swings and water quality can deteriorate rapidly due to the small volume of water....

At 1kg of size ... with 15 fish in 500L.... each 1kg fish would be living in about 33L of water... and excreting their wastes into that same volume of water....

Good stuff and a very helpful indicator of stocking rates per lt GBs