Pump is drawing air in wet/dry filter

THE PROBLEM:

The filter pumps/water flow isn't reaching equilibrium. The right side pump seems to be emptying the sump faster than the left. Even with both pumps adjusted down to the minimum power the right pump is drawing air.

I have gotten the pumps to work together for only a max of 12hrs at a time before one side starts drawing air (usually the right) or the castles start to fill up. I've even resorted to blocking some of the openings at the top of the left tower in case the location of the water returns (in front of the left tower) was causing part of the problem.

THE DETAILS:

I have previous freshwater tank experience with both 125g and 150g tanks, so when we were given another large tank, filtration and all, I thought it would be no problem. Hah!

The new to us tank is 6'x2'x2' and it's my understanding that means it's 180g. It was a saltwater tank but we are intending to do freshwater with it. At this point, I can't get the two filtration pumps to find an equilibrium so it doesn't really matter what kind of fish we will eventually get for it.

I'm new to the wet/dry filtration system that came with the tank. It came with two larger (unmarked) powerheads that didn't seem to be able to keep up with the natural drainage and flow of the tank. So we checked (individually) the gravitational flow from the castles and determined it was 240g/hr out of each one. We didn't test them at the same time. Then we called a reputable local privately owned fish store and was directed to the Marineland Maxi-jet submersible utilit pump 1800 (475gph max/255gph at 4'). I did tell the fish guy we were running two filtration boxes and needed two pumps.

Our pump outlets (returning the water to the tank) are on the left shorter end of the tank, in front of the left tower.

 

I have a few questions:

1. Is what you're describing "overpumping" your return so that the pump(s) is sucking air due to insufficient water level?

2. Altho it appears that you have two pump bays (one on each end), does the water from the trickle tower "translate" between the pump bays once the water has trickled down, or are they totally separate from each other via baffling?

Running twin pumps can be maddening, as they're virtually impossible to "tune" them to match flow for any length of time (as you're finding out). Your best bet, assuming the water in the pump bays is "common" at that point, is to run a single pump.

If you're "stuck" with using twin pumps, your best bet is to run T fittings and bleed lines back to the pump bays, as this will allow your pumps to run flat-out, but will return any "un-returned water" back to the sump so it won't go dry on you and cause bubbles:

 

I'm a bit confused on terminology...are the castles and towers the overflow compartment in the tank? Do you have two separate overflows in the tank, each one feeding into a different wet/dry filter?

If you need two separate compartments/filter boxes, have you tried building a siphon between them to keep the water level in them the same regardless of the overflows?

 

We are stuck with the twin pumps at this point because the trickle castles drain at each corner directly into the separate sumps. Unless we can T them together into one sump and run one pump. Hmmm...

I'll talk to my dh about the T fittings. That sounds like a fix he can do. Thanks for the diagram. That will help us both be on the same page.

We haven't tried the siphon but I'll talk to my dh about it. It sounds like creating a connection between the filter boxes is our best bet.

Thanks so much for your feedback and suggestions.

 

For a 180 gal tank that's not a lot of water movement. 475gph

I run 1200gph through my 180g with the same footprint. Single pump

 

I don't see a standpipe in your overflow box, that is a major part of your problem.
Second, never return water to the sump via a ball valve such as Greg suggested, this is very inefficient and harder on the pump and uses more energy than placing a ball valve on the discharge side of the pump, inline so it can throttle the return flow back to the display. What most people don't understand is cenntrifugal pumps are in no way harmed by this and in fact use less energy since they are doing less work than if you were returning wasted flow to the sump. A simple Kill A Watt meter will prove this out. It is taught in school in Bernoulis Theory and the Laws of Affinity.

The standpipe will allow the correct mix of water and air for a laminar flow to the sump. You will also want to remove the ball valves on the overflow piping or open them completely except for maintenance where they can be closed completely, never try to adjust the overflow rate, it is determined by the return pump flow, never with a valve on the gravity to the sump.

Everything you would ever want to know about sumps is here:
http://www.melevsreef.com/allmysumps.html