Posted: Today at 08:23
Not sure what sort of filter you're using, but I'm guessing it's a box filter filled with some sort of sponge or other surface that bacteria can live on, with a built in UV tube.
These filters take about 6-8 weeks to build up the bacteria colony so that they can start turning toxic chemicals into harmless chemicals.
The UV filter will kill the algae as it passes through.
But it all takes time to settle down. Don't be in a rush and don;t clean the filter too often. I clean mine once every 3-4 months.
When cleaning the filter, you should always do it in stages. You don't want to kill all the bactiria that have built-up, so clean the filter in stages over a couple of weeks using tap water. The chlorine in tap water will kill the bacteria, that's why you need to do it in stages.
If you choose to clean it all in pond water, you can do it all in one go.
If you have blanket weed, you need to use Cloverleaf or similar.
If I've got the type of filter wrong, please let us know.