Have you added a new filter.
If so squeeze some bacteria onto the news sponges off the old filter sponges, Soak them in tank water first.
I tried to buy a new filter but my LFS asked me why, so I said that I am overstocked and just needed a new filter to cope. Then he asked me about my weekly maintenance and how I clean my filter sponges.
Q and A:
Q: How much water change I do?
A: Once a week 20 litres, every Sunday/Monday.
Q: Do I clean the gravel and filter sponges?
A: Yes, everytime I do water change.
Q: Do I clean the filter sponges with tap or old tank water?
A: I clean the sponges with old tank water.
Out of these he thought that I might be over cleaning my tank. And his avice would be to:
Do 5 litres of water changes with gravel cleaning twice a week, that's a total of 10 litres a week.
Change the white sponge every 2 weeks, every week if required.
Change the black or carbon sponge once a month.
Clean the Nitrate (green) sponge once in every 3 months.
Clean the blue sponge once in 4-6 months.
Then if I still don't see any difference in my water readings then I can come back and get the filter. But he was quite sure I am over cleaning my tank and he strongly adviced me to try the regime first before spending so much to an additional filter. Does anyone thinks the same? Or is he totally wrong?