What do you mean "they were only red devils"? Are you implying it doesn't matter if you kill a fish if, for some reason, you consider it less important than other species?
I get angry with people who don't care if they have killed a fish because it was only cheap, they had lots of them, didn't really want that fish anyway, it was given to me etc etc etc.
If you keep fish you must house them and care for them to the best of your ability. They are reliant on you for their survival.
If you had a litter of kittens, or puppies, there would be a fuss made if it was discovered you had managed to kill most of them because of something you did and then you said "It doesn't matter, they were only moggies". Why should fish be any different? Because they can't complain, don't look cute?