I've noticed that even some of the fish that come from the basic eggs aren't suited to a beginner tank. They'll generally survive long enough to become adults so you can sell them, but they don't make for good breeding pairs. Stick with the hardy ones.
Pay attention and you'll notice some patterns. Some fin types sell for more than others, no matter what fish body they're attached to. And there are fin types are less hardy and no matter what body they're attached to, they won't survive long.
I don't do advertising and tank decorations at first. I concentrate on getting good fish that'll be hardy and bring good prices before I worry about that. That way I'll have steady money getting in to keep funding things, even though I'll have to sit for longer in the store while my customers dawdle around.
Click on the customers - some of them will show you what they think of your prices in a thought bubble. If it's taking ages for things to sell, price them lower.
There's a difference between "sick" and fish that just aren't healthy. If they're sick, people won't buy them. But you can sell a fish whose health is all the way down in the red zone. Price it low so it sells before it dies.