Aquariums: What’s the craziest thing you’ve found when doing filter maintenance?

I just removed one of my big tank’s two FX6 canisters to clean it and used the utility valve to drain it into a bucket. I always use a bucket rather than going straight into the sink, partly so that I can use the water on plants outside, and partly to makesure that if there was anything alive in the filter, it gets rescued. By ‘anything alive’ I had shrimp or possibly guppy fry in mind. I was expecting shrimp or possibly fry. I wasn’t expecting a 2 inch bristlenose!

Bristlenose plecos have been breeding in that tank and one of the fry must’ve gotten past the sponge filter on the intake and grown up in that canister. It seems perfectly healthy.

Prior to this, the most exciting thing I ever found was when I was upgrading my 40 gallon to a 75 and moving everything over to the new tank. Some months back, I had bought a group of black Kuhli loaches and never saw them again. I didn’t find them in the sand or under the rocks when I was transferring everything to the new tank, so I figured they must’ve not made it and been scavenged before I ever saw bodies.

I removed the Aqua Clear 70 filter and took it to the sink to clean it before putting on the new tank, and when I started lifting the sponge basket out, chaos ensued. Loaches exploded in every direction and I yelped and dropped the whole filter into the sink. The filter surprisingly wasn’t broken, and the loaches – all six of them – were captured before they went down the drain. All six are still alive several years later. Every time I do a filter change on that tank, though, there’s still that moment of fear when I take off the lid.

I’m sure this sort of thing isn’t rare, and I’m curious what ‘filter surprises’ other people have had?