Inadequate filtration?

I've run a HOB filter ( 2 filter bags, 2 sponges) in my 55 gallon for
several years. I am setting up a small tank for someone by runnig his
sponge filter in my tank for a couple weeks to minimize the cycle
time.... The sponge contained more of a biological mass than I'd
expected. Ya' think this is a sign of insuffitient filtration in my
tank? I know that bacteria will grow on anything that holds still long
enough, but I actually had to clean the sponge at about 10 days.

TIA

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