Friendly Bacteria

What is it and how does it work?

Friendly bacteria change the harmful ammonia (from fish waste) in your pond water into nitrites and then a different type of bacteria turn it into nitrates, which go back into the pond to feed your plants. The friendly bacteria are present in the pond naturally but the filter creates ideal conditions for them to thrive.

The number of bacteria in a filter box will be determined by the amount of ammonia that is fed to them (among other things) so if a lot of ammonia goes in then there will be a lot of bacteria. It takes time for the bacteria to multiply to match up with their food supply so if the amount of ammonia goes up quickly then it can be several weeks before there are enough bacteria to deal with it all. This is the source of the problem known as ‘new pond syndrome’ (or new tank syndrome to those keeping aquarium fish), which happens when too many fish are added to a pond at once.

Commonly people set up their pond and want lots of fish in there quickly. They are interested and so feed the fish often which is exactly how new pond syndrome starts. Then the fish get sick and die so the new pond keeper waits a few weeks. This seems like the sensible thing to do, but unfortunately it is just long enough for the ammonia to disappear entirely and the bacteria numbers to drop back down to background levels. Then they do it all again and their fish die again. Then they fill in their pond and build a rockery!

This problem is really easy to avoid. Build the pond, set up the pump and filter and then wait a week or two. Then add fish very gradually. A typical garden pond with, for example, 2000 litres should start with 2 or 3 goldfish. Give them around three weeks and add 2 or 3 more. If you add fish in this way then the levels of ammonia and nitrite will never get to a point where the fish will be endangered.

There are ways to reduce the level of ammonia if you find yourself with this problem, such as partial water changes, chemicals that ’lock up’ the ammonia or bacteria that can be added to speed up the maturation of the filter, but it is much better not to get to this stage simply by being careful when adding fish.

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Now that you know a bit more about friendly bacteria, why not take a look at one of our other guides?
Waterfalls
Pond Filters
Fountains
Box Filters vs. Pressurised Filters
Connecting the Pond Pump and Pond Filter
What size of pond pump and filter do I need to buy?
Filter Maintenance
Adding fish to the pond

Or click here to see a list of all our pond building guides