It all depends on your goal mate. I've made a bit of cash selling bred fish, look around the LFS if you want to do that and see what is a good price on the shelf, and if you can go and breed them. As I said before The Endler Tiger Guppies are really popular and really easy to breed and go for a good price. If you want to sell them then you have to minimise your losses, so a breeding tank in the main tank is good to mature them a bit before letting them into the main tank, or you could buy a nano tank and set it up just for bred fish, or to breed fish that may not make it in some tanks.
LOL if you started with a couple of dozen guppies in a tank, with good plants for the fry to hide in you'll have hundreds by the end of a year! if you have a good quality a lot of shops will buy them, but they have to want them!
Somethings are easier than you might think and sell ok, like rams, Cory's, any live bearer. Check out this guy...
He's really good at breaking things down, and he isn't "shy" with his methods. Probably a good channel to give you an idea or two.
But if you really want to keep them all get something like this..
Thats just an example of dozens, some stick on with suckers, some clip on the side of the tank. Some like this one which I prefer floats. As I say give them chance to be beyond lunch size before releasing them. I love it.... it's great fun watching the cycle and seeing the little wrigglers come from eggs, or live ones been born. You can spend hours just glued to the tank. I think you have to be much more careful with egg layers as they are like Macdonalds for a lot of fish! But again you can move the eggs if you're careful and put them safely in a breeding environment. Some fish just lay their eggs and clear off... like Cory's, other fish like Rams are very protective and can be territorial and aggressive, but it only takes one time for both parents to leave the hatching spot, like half a coconut or a little cave where they can keep the fry safe, and thats that! I know I probably sound technical a wee bit, but I sincerely don't believe we have tank systems... We must have tank habitats. They're little lives and we are responsible. Hope this helps and I hope I'm not teaching you to suck eggs... apologies if I am. But as with anything... research, and then research some more. Good luck BigJ... and welcome back. Don't get bogged down with technicalities it's all about enjoyment, and peace and a great hobby with great people around in it. Kia Kaha. Phil.