"poop" filters as someone once remarked
Same here, have small PHs on 40 gallon rated sponges running in two 20 longs and 2 40 breeders (these latter in tandem with small pre-filter equipped HOBs - one for Tangs*, the other for Corydoras/Apistos). Creates nice circular flow in the tanks so all the detritus accumulates around the sponge filters, but they do need to be squeezed clean every 2 to 4 weeks or they tend to collapse because they get so clogged with waste that the suction just draws them in on themselves. If you can deal with that as a maintenance schedule, it's not a bad way to go. Would recommend it provided you don't use a sponge any smaller than a 40-rated, but guessing you may be going up a size given that it's a 90. Haven't yet tried sponges on large tanks except as auxiliaries but as mine are all mostly overstocked am doing the twin canister thing. Does seem like it would be hard to get the full benefit of a large sponge without running it off a powerhead unless one just wants to go months between cleanings. You can also stack sponges on a common and longer uplift/intake tube which I have never tried but that might work well enough to preclude the need for additional filtration - might be a good way to go for a 90 - pretty sure Jehmco has all you'd need at great prices.
*Am sure someone is thinking that Rift species don't require/benefit from current but mine seem more than merely content.
