I'm in the process of adding the fun parts to my world, and I'm a bit perplex on this one;
Let's assume a fantasy ooze
An ooze is a mindless carnivorous blob of jello - think cube-shaped or formless jellyfish. It has no natural predators, and feeds passively - if organic matter touches it, it gets sucked in and digested until there is no trace left. Oozes are quite slow, and the bigger they grow, the slower they get. Main way to tell different species apart is by colour, but the danger resides in the fact that they tend to blend in the environment.
Let's say an ooze grew so big it became mostly stationary and sipped in a lake, filling the bed of it from the bottom. It is translucent-blue, so it blends very well in its environment. Water has to flow, so there is still a ~2m layer of it above the ooze and thus, the lake in itself is not fatal. The locals learned not to skinny dip in it, sometime a child goes missing, but overall, people and the ooze can coexist without much problem.
But what about aquatic life? Obviously, there is no algae in that lake below a 2m depth. I'm curious regarding migrating fish species making their way upstream for reproduction. Would they evolve to avoid that very specific lake? Would we see any behavioral changes over a couple of centuries?
We are talking regular biology and animal behaviour here. The ooze in itself is a magical creature and heavily handwaved. We are not discussing the potential ecological problem in a few millennia when that ooze grows so big it begins to destroy the ecosystem if not dealt with. We are not discussing how the ooze is biologically unsustainable. I am just asking what the fish will do when confronted with an unusual, localized area that is highly hostile to them in their migration path.