I'm having some issues with seemingly never ending high nitrates in my 3 betta tanks. The problem started about 3 months ago, I thought I'd dealt with it over a month of daily water changes but seems to be back again. I have two 38L tanks and one 68L tank, each home to a single betta, live plants and some red cherry shrimp. The larger tank was a community tank but a few weeks ago I switched out all the fish for a single betta. Each tank is heated and filtered. I run two sponge filters in the smaller tanks, as well as a Hagen HOB filter on one tank to deal with some detrius issues I had in that tank. The larger tank runs an Aquaclear 50. I change a minimum of 25% of the water in each tank once a week with a gravel vac and Seachem Prime. Once a month I do a larger 50% water change. I've followed this routine for the last 4 years but have only just started having nitrate issues. The smaller tanks have been running for 4 and 2 years respectively, the larger tank for around 10 months. Last night the betta in the large tank jumped out despite it having a cover, I think he got out around the HOB filter. I'm not sure if it was the nitrate levels or just bad luck that caused him to jump but he didn't make it. Aside from this and losing another couple of fish recently after having them 1.5 years my fish seem active and eat well. The nitrate levels in each tank today are between 40-80ppm, despite yesterday's 25% water change. My tap water is usually 0ppm but sometimes it's 5ppm. My API master kit is not expired. What could be causing these constantly high nitrate levels and what can I do about it?