I installed an external filter two months ago and test the water every week. Nitrites are sometimes normal, then they spike to dangerous levels, nitrates are above 40. I do 30% water changes weekly. Maybe the filter isn't powerful enough for my tank, or am I missing something?
A filter catches organic matter, but nitrites and nitrates are the result of organic decay in the tank, so filter power isn't really the main issue here. Most likely you're overfeeding your fish or don't have enough plants that consume nitrates - check if you're putting in more food than they can eat in a couple of minutes, and think about getting live plants, especially fast-growing ones like ludwigia or cabomba. Nitrites also spike if organic matter gets trapped in your filter - clean it more often, don't wait until it's completely clogged. And even though 30% water changes weekly sounds decent, with these levels try doing two smaller changes instead or one bigger one.
Don't overcomplicate things with filter power - that's definitely not the issue. Nitrites spike because your tank's balance is off: too much organic matter (overfeeding, dead plants, decaying décor bits) and not enough stuff to process it all. Nitrates above 40 - that's also a sign your bioload is too heavy.
First, check this: do your fish eat all the food within 2-3 minutes or does some settle on the bottom? If they're grabbing it right away - cut back on portions. Then look more carefully at your plants: they should be noticeably slowing down nitrate growth if there's enough of them and they're healthy. Maybe your plants are struggling from poor light or nutrition, so they're not doing anything useful? Also check if there's rotting organic matter in the substrate - stir it gently once in a while.
Check whether the filter is installed correctly and isn't clogging with organic matter. Sometimes the housing doesn't seal properly or the tubes get kinked, and then water bypasses the filter without being properly cleaned - it looks like it's working, but in reality most of the dirt stays in the tank. Also, take a look at your circulation pattern: if water in the tank isn't moving evenly and stagnates in certain corners, organic matter there will decompose more actively. Add dead zones without flow to your overfeeding and plants.
I ran into something similar in a neighbor's tank - nitrites were spiking even with a filter and water changes. Turned out the problem wasn't just one thing, but a combo of several factors. Yeah, overfeeding and plants matter, but there's another thing people often miss: the bacterial colony in the filter itself can be unstable. If the filter is new or was recently cleaned, the microorganisms that convert ammonia to nitrite and then to nitrate haven't really settled in yet. The spikes are exactly about that - when there aren't enough bacteria, they can't handle the load.
Also, check if the filter's clogging up too fast. If blockages happen frequently, it means there's actually a lot of organic matter, and the filter just can't keep up. But here's the main thing: two weeks for stabilization is too short - bacterial colonies take roughly a month to a month and a half to establish. With 30% water changes you're doing good, but make sure you're not overfeeding. Fish should finish their food in 2-3 minutes, anything left goes in the trash, not in the water.
Nitrates above 40 are fine too, as long as you're not hitting 80 and above. They build up over time and you can only get rid of them with water changes and plants (if you have fast-growing ones). Just keep doing your weekly tests for another month, check your feeding schedule - that's usually the first thing that makes a real difference.
The biological balance in an aquarium takes months to mature, and two months is still pretty early for stable nitrifying bacteria work 😊 They colonize the filter and substrate slowly, so nitrite spikes are totally normal at this stage - even with ideal conditions. At the same time, check that you're not overfeeding and that you have enough plants to consume the nitrates, but the main thing is just give the system more time to cycle, keep doing water changes, and don't panic over every test result.
I'll add: nitrates over 40 is already a buildup, and water changes alone won't get them out even with regular maintenance 😐 I can share some experience - I knew someone in a similar situation, and when he bumped up water changes from 30% to 50% a week and removed some décor that turned out to be an organic source, the levels stabilized in a couple weeks.
But the main thing - make sure you're not actually overfeeding. A lot of people feed fish by eye and don't notice how much food ends up on the bottom and in hiding spots. Plus check that your filter is actually doing something: sometimes it looks like it's running, but the biological filter (bacterial colonies) just never established or died off from parameter swings. If your filter's only two months old, it might not have ramped up to full capacity yet 🚴
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