It is the same. it doesn't matter if they are in a lake or in a tank, nature decieds if they will mate and what becomes of them. MAN will deciede to keep the bloodlines pure. I do think some should remain pure but killing hybreds just because they are is stupid and it is playing god, who knows what beautiful breed might someday come from them. Nature has made EVERY type of life on this planet from hybreds, nothing is like its first ancestor was. When I think of ridgidpurity of the species the first thing that comes to mind is Hitler and the KKK. I know this is fish but they have a right to live and evolve.
This is wrong is so many ways.....As soon as you take the fish out of the lake and into a tank, it's no longer natural (nature), the instincts of the fish (nature) will drive them to mate, but they will need to un-naturally go against their instinct to choose a mate of their species since a suitable mate is not available. In this situation the more primitive instinct to mate must override the more recently evolved instinct to select a mate of their species. Nature has most definitely not made every type of life on this planet from hybrids, very very very few species are a result of hybridization. Most species are a result of geographical isolation, sexual selection (genetically based preference for some specific quality in the opposite sex), genetic drift, and/or similar selective pressure. Hitler wasn't looking for rigid purity, he was interested in selective breeding of humans for certain qualities similar to what we do with livestock/dogs/cats,
and he had a deep hatred for jews...those two things aren't connected in the way your thinking. The "playing god" argument is not great, it has been used for almost everything: using medicine/surgery, birth control, spaying/neutering your pets, breeding livestock, killing a bug, telling your kids what to do or not do, etc.
Sorry to go off like this, but opinion and false information are different things. I'm fine with opinions, some of them are wrong

, but people are entitled to them. Putting forth false information as truth is unacceptable.