So what I understand is that the sponge filter will still work for 60 gallon even when underpowered?
Hello; yes if some additional measures are taken. First and foremost will be the use of waterchanges (WC). There is an addendum or two concerning the WC also.
An air powered sponge filter provides agitation of the water surface from the bubbles breaking the surface. This aids some in keeping the oxygen supply stable in the water. First the agitation stops a protein film from forming on the surface. Next is some improvement of the gas exchange at the surface and in the water column with the bubbles. Back three or four decades when the early powered filters started to be available at affordable prices they were prone to stop working quite often. The relied on a siphon which could be and was broken easily. Not like todays filters which can self start after an interruption such as a power failure. The old stuff did not lose prime every day but a few times a week was not uncommon. I was in the habit then and still am of running some sort of a bubbler in all tanks. A simple bubbler can and did save my tanks many times. (Note - I was still overstocking tanks back then and an overstocked tank is into trouble very soon after a filter failure.)
A simple sponge filter is fine for biological filtration. The bb (beneficial bacteria) can form colonies on the sponge filter parts but also will form colonies on other tank surfaces. The air bubbles help by creating some current, weak but still current, which moves theater over the bb colonies.
A sponge filter is very poor at mechanical filtration. Some detritus (mulm) will be trapped in the sponge itself but none is actually removed from the tank. This is where the WC come into play. I try to siphon out detritus (mulm) when doing a WC regardless of the filtration but with an underpowered filtration system the WC becomes much more important. With a less that ideal filtration system you can still run a healthy tank by doing sufficient WC.
How much WC? That is a subject of much debate and argument. In fact even with the biggest and best filtration systems you need to keep up with WC anyway. The subjects of WC volume and frequency are debated often on this site . Plenty of past threads on that. How much filtration is enough is another hotly debated topic. I was lucky when I found this site almost ten years ago that I had several decades of practical experience behind me. I already knew what could work.
As you may have already guess there are many layers to this filtration stuff. Some will depend on what sort of fish you plan to keep. Some are more messy than others. How much stocking density will you have is another. Tank volume is indeed a part of the equation but I have had to run bigger tanks with less than optimum filtration several times. Sometimes you have to make do with what you have or can afford. Right now I cannot run down to a fish shop but I have a few extra air pumps laying around so know I can keep my tanks going.
Good luck
