Backyard Aquaponics
A place to discuss aquaponics

I have some 4" PVC I can use for the drains for the Bell siphon so I think that problem is fixed, but I noticed a bigger issue...

Each grow bed is going to hold roughly 400 gallons of water (if empty of gravel) if the gravel takes up 66% to 75% of the space in the growbed that still leaves over 100 gallons of water for each growbed (times 3) to be pulled from the 650 gallon fishtank if they happen to all fire at the same time.

You will need to keep an eye on the FT water level or you may end up in a situation. Float switches are a good idea, so is elevating your pump from the FT floor to prevent pumping dry. You could flood all the beds and see how much water you have left in the FT, if your not comfortable with the remaining water then pull out a syphon or two and run them CF.

And there can be a lot of water losses each week when it is hot due to evaporation etc 100s of litres depending very much from system to system.. I have a lot of sump capacity and 2x200L barrels for auto top up... sometimes I lose over 200L a week in those barrels. Also if your water level fluctuates (lowers) it changes your head height and therefore the amount of water flow.. this may effect your bell siphons? I have never used them but I thought they could be sensitive to flow changes? maybe someone else here knows?

I agree my other systems fluctuate a lot depending on the weather. I think I have to pump the water from the bottom of the tank to to RFF filter and let it gravity feed to the growbeds if the fish tank is going to vary by 1/2 the water. OR just get rid of the RFF filter and let the bacteria and worms take care of the chopped up solids in the growbeds. I can get a pool style basket prefilter from aquatic ecosystems for about $48 to save my pump from hard solids that fall in the fish tank.

Can i ask how have you plumbed your top up barrels ?

They are just connected with irrigation polypipe from bulkhead fittings at the bottom of the barrels, then they flow through one pipe to a float valve in the sump

like this..
also there are ball valves so I can isolate each barrel if needed... for no good reason appart from the fact I love putting them everywhere.. along with barrel unions