Are you talking aquarium or trout pond?
If you're talking aquarium, the first 4-6 weeks are the roughest while the tank goes through the nitrate cycle. Resist the urge to add a bunch of fish before the tank is fully cycled and the toxins have leveled out. It gets expensive and frustrating to try and keep a bunch of fish alive in a cycling tank.
Depending on the size of the tank, a couple Platys are excellent for starting a new tank. They are hardy and can survive the toxins if you keep it in check. Try to get 2 females because Platys breed like rabbits lol.
Once the tank is cycled you can put whatever you want in there and then just adjust the water hardness & PH to match the species.
It can get pretty pricey buying all your test kits and what not at the beginning, but they last a long time.