Wandering fish
Do you ever find fish in weird places? Like they migrate between tanks when you're not looking?
This Ancistrus sp. L509 was a juvie from a group that I moved out back at CatCon 2022 (more than a year!). He just turned up in our C. saulosi tank. No idea how he got there.
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May have been hiding inside a sponge filter and you moved it to another tank. Happened to me before with some Loaches.