I’ve had a few people ask about doing constant height in Fish tank systems with Aquaponics indexing/sequencing valves lately.
Doing constant height in fish tank (CHIFT) with an indexing valve does require a slight modification of thinking. There are ways to do it but I don’t think I would recommend doing it with only one pump. (It can be done but requires some complex plumbing and automated valves and probably isn’t worth it unless you really want to tinker with it to get it working properly.)
Why not just branch off from the pump to feed the fish tank and the aquaponics indexing/sequencing valve at the same time?
The reason is because the indexing valve requires a certain amount of pressure and flow to operate and if you T off to feed the fish tank before the indexing valve, you may not get enough flow/pressure to the valve unless you are very careful about choosing the pump and balancing the flow to the fish tank. I’ve not tried it this way so won’t guarantee it would work. Now if you are providing plenty of flow to not only flood the grow bed but be able to plumb back some of that to the fish tank it might work but sounds like a pain to me, extra plumbing and all.
What I would do is a spider CHTP (Constant Height two pump) system. Get one pump sized correctly to feed your sequencing valve and flood your grow beds in the time frame you wish. There are repeat cycle timers out there that can do what you want, they just cost more than the mechanical 15 minute increment timers. I’ve got one running 9 minutes on and 1 minute off so each of my six grow beds gets flooded once an hour. Then I would get a second pump sized appropriately to turn over your fish tank volume as much as you want and provide constant flow/aeration to your fish tank, I would even probably put this pump on the battery backup. Depending on the system configuration and the pumps in question, you might even have both pumps be the same model and then you could keep a spare as a backup that would work in either position if there were ever a failure.
As to the size of the sump tank for such a system. I would make it big enough to flood at least 3 or 4 of the grow beds at once (I am assuming flooding one grow bed per outlet of the indexing valve, if you are flooding more per outlet then the figures need to be for the entire outlet not per grow bed), this should give some leeway for pump height an a top up valve. Sizing the flow rate and holes in the stand pipes for this can be a bit of trial an error though. But I would expect that there are usually about two grow beds worth of water out of my fish tank at any one time on my 300 gallon system that is running the pump 9 minutes and one minute off. One grow bed is filled at the point the pump shuts off and the previous grow bed has usually drained about half way and other grow beds are usually still trickling for quite a long time. You don’t really want your sump to overflow if the power goes out and all grow beds drain completely.