Backyard Aquaponics
A place to discuss aquaponics

first off hello to all...i am reading an aquaponics book and using the 15/45 fill drain i do not understand how if your pump does the volume of the tank in 15 minutes how you aren't killing the fish by draining the tank? also why then if you had a 100 gal tank would you need a 400gph pump, wouldn't a 100gph pump do? i just don't understand how the math adds up and the author of the book was pretty vague on this...i would imagine full grow beds for 15 mins and a completely dry fish tank with fish gasping for air...can someone explain this better for me

G'day drane - mate, for the above question, you need to ensure that your fish tank has a greater volume than your grow bed(s). Remember, the grow bed drains back into the fish tank, so if the fish tank holds a greater volume there is no way you could empty it.



Well, the gph stands for gallons per hour. Obviously if you put a 100gph pump in a 100 gal tank, it's going to need an hour to actually pump 100 gal. If you put a 400gph pump in, then it moves four times as much water in an hour, therefore you only need a quarter of an hour to turn over 100 gal.

Clearer ?

are you filling for the whole 15 minutes or say in 5 min increments, because to me no matter the size of the tank its still being drained to move that volume regardless of the grow bed size...if you had a 100 gallon tank and needed to pump that volume in the 15 minute fill cycle you drain the tank if its all at once...i get the gph now but the volume question i still dont understand if its only filled once...if you have a 250gal tank and you fill 100 gallons in 15 minutes that is not the volume of the tank...i'm not understanding this

What book is it?

Depending on the design, the water will be pumping up, but then also returning to the fish tank at the same time...

if your tank is 100 gal and your grow bed is 50 gal, the whole hundred cant be in the 50 gal grow bed all at once can it ?

Also note that a media grow bed approx holds 45% water. So using Chilli's example, a 50gal grow bed would hold approx 22.5gal of fish tank water because the media itself takes up the remaing 55%. If pumping out of a 100gal tank at 100gal/h it would take approx 15 minutes to fill and you would still have 77.5 gallons remaining in the fish tank.

Gee, I hope I got my maths right on that one

This Gallons stuff is so confusing....

Oi.... catch up with the rest of the world you lot....

Countries still using Imperial system

lol eb.

Drane, in these 15/45 systems, the growbed gets filled and drains back to FT many times in that 15min.
so if you have 100ft and 50gb, then the ft will quarter empty - see chariles ratios above - then the syphon will kick in (or the stand pipe will overflow), this will continue to happen for the next X minutes, until the pump turns off.
with a syphon, the FT will raise and lower, but no more than half (or less because of media), with a standpipe with hole, it will drain to the low point, then sit there until the pump turns off, then slowly fill again as GB drains.

Drane, also keep in mind the whole setup is an enclosed recirculating system, so there is nowhere for the water in the growbed to escape to but back into the fish tank. No water is wasted or leaked out even if you have too big a pump.

eg an extreme example:

100 gal fish tank
50 gal growbed
1000 gal pump (way oversized)

Even in the above scenario, no water would be lost, nothing would overflow onto the ground, you would just have an overly high water turnover and higher than needed power bills. Obviosuly in real life you would use a smaller pump better suited to the job.

Also keep in mind the growbed is not 'flooded' to overflowing the sides, but there is a hidden standpipe in the growbed where the water drains down through back into the fish tank:

i was assuming it was filling but not draining back, hence why i figured the tank would drain...thanks for clearing that up...i am totally new to this and i want to understand all of this before i set a system up. the way the book made it sound was it was a filling cycle and when adjusted properly and the fill cycle being complete, then the system would start to drain. i had no idea it drained as it was filling. thanks guys for the help

One other thing I'd recommend, have a sump tank to pump out of.

So the water overflows from the fish tank, into the sump tank. So the fish tank never changes it's water level, it's only ever the sump tank that fills and drains. So if something goes wrong (like overgrown roots blocking the return pipe, or burst pipe), it'll be the sump that's drained, not the fish tank.

^ that setup only works really well if your beds are lower than your FT and you are forced to.
If not, then you are liquidising your solids and probably not catching as much as you can/should in your GB.