I just have goldfish.
There’s a plug in the bottom of the tank; I can drain it that way. Once it is low, then it can be flipped and hosed out.
I have a net for catching the goldfish. Some of them have gotten pretty big. We scoop a bucket of water and put them in there while we change the water, which we don’t do all that often. I do obviously fill it and top it off regularly, and skim anything icky off the top.
We have some pieces of PVC pipe in the bottom for the fish to hide in, to keep them from being a wildlife buffet, though we do lose some over time.
It gets very hot here and sometimes the horses knock over lesser waters. In winter, buckets can freeze effectively solid but the tanks do not. The 300 gallon tanks stay put and there’s always water in them. The water isn’t pristine but it’s good enough that when I turned out my mare from her pristine buckets-in-stall, the first thing she did every morning was mosey on over for a nice drink of fish water. The horses like bonking their noses around in it and playing a bit, sometimes chasing the fish, other times just enjoying the splash.
And, my daughter has been known to, ahem, ‘inadvertently’ play in the tank as well.
My horses have survived this way for years.
It is far more dangerous to them to be out of water than to have it be fishy.