In an overall healthy pond environment, it would be difficult to add so much bacteria that it’s actually harmful to your fish. It is possible, however, to create a situation where the bacterial load is so high that it consumes all the available nutrients or depletes the oxygen to the point where most of the bacteria die. Potentially, this will leave you with an inadequate bacterial population, and if you try to compensate by overdosing with bacteria again, you could find yourself in an expensive and stressful cycle.
Beneficial bacteria is quite prepared to take up residence in your fish tank, given time. Brand new ponds and aquariums can generally develop their own bacteria colonies in just a few weeks, just by sitting around fishless, with a few plants. If you’re eager to hurry up and populate your pond (who isn’t?) then you can give it a running start by adding a dose of bacteria from your pond supplier. There’s really no need to add more than the recommended amount, since bacteria can multiply quite rapidly in good conditions. You might supply some extra aeration to ensure that there’s no shortage of oxygen while they’re getting established, and the colony will naturally establish its own equilibrium without interference from you. Of course, if there’s a crisis such as low water levels that leave your biological filter dry, that will kill most of your bacteria and you’ll likely need to take extra precautions with water quality while it builds up again. If your pond is polluted with chemicals like chlorine or ammonia (perhaps in a misguided effort to clean your pond of algae), you may be facing bigger problems than just the extermination of your bacteria colony.
