Adding Fish to your pond while maintaining optimum water quality
You should let your pond mature with the plants in for a week or two before you start adding fish to your garden pond, to ensure the pond has the best chance to obtain the correct pond water balance.
Once you have left it to mature or at the point where you think sod it, it’s my pond and I want fish (think it took me about 4 days), it is essential that you perform a pond test. This will check that the conditions of the pond are OK for fish to survive in. Once you have done this and the water is a good balance you can start adding them slowly, maybe 2 or 3 to start with then gradually adding more over the weeks. All the time performing regular pond tests to ensure the balance of the pond is still acceptable.
If you get a reading that is border line OK then do not add anymore fish until this reading has recovered and the pond is balanced again. If this figure does not correct itself you might have to look into how to solve the problem.