This may sound like a stupid question (it probably is). The PVC pipes, maintain water level in all the tanks correct? Does the HOB filter on the far right have any filtration effect on the tank all the way to the left? I assume the sponges are more than enough anyway..but if you wanted to try something like that without the sponges would it work. (assuming 2x per week water changes from the filter tank, thus pulling the less filtered water to that tank and then when new water is added leveling things back out)
Yes, the PVC helps keep the water levels the same when you suck the air out of them. To some extent, each filter affects the water in all the tanks. Each tank has a HOB filter on it. There is a internal filter in Tank 1 that distributes water to Tanks 2,3,4. This forces Tank 1's water level to drop as the internal filter pumps water out of the tank. This forces water from Tank 2 into Tank 1 to keep the levels the same, then from 3 to 2, 4 to 3, then from 1 to 2,3,4 and the whole cycles keeps going so all the water is same and being moved around from the tanks. Because the spraybar from the internal filter isn't distributing the EXACT amount of water to Tanks 2,3,4, their levels will be a bit different and might have to pull water from 3 to 4 to keep tank 4 level if it's getting less water, etc, so basically each filter will end up filtering the total water volume in some way.