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I have a 29-gallon planted tank with mollies, mystery snails, and several live plants.

One of my plants has been showing signs of yellowing leaves: enter image description here

I use Seachem's Potassium, Nitrogen, Phosphate, and Flourish liquid fertilizers in the specified doses twice a week or so. I also use Seachem Flourish root tabs, and recently added more of those. However, it doesn't appear to be helping. I don't use any kind of carbon dioxide supplementation.

I use white LED lights. The aquarium gets indirect sunlight. Right now nitrates are higher than usual at around 25 mg/L (I'm working on reducing that, they're usually quite low; in the past they've sometimes been pretty close to zero) with zero nitrites or ammonia. Total Hardness is on the high end, somewhere between 75 - 150 mg/L.

Does anyone have any suggestions of what the problem may be, or of other things I could try to help the situation?

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  • never give nitrogen fertilizer in a tank with fish or other animals.please tell a bit more about the led lights,colortemperature and how many watts,how deep from the surface to the gravel in cm? Commented Dec 7, 2022 at 7:27
  • @trondhansen I'm struggling to find the exact specs for the lights - they're the standard Aqueon LED 29 Gallon hood. The tank depth is 20.26 inches according to the manufacturer, and I have around an inch of gravel or so. Commented Dec 7, 2022 at 14:29
  • The picture you provided is quite strange, from several points of view. Will you please upload another, better picture? Also, please clarify if you have algae problems - I seem to notice spots of them around. If you have algae, you probably need to concentrate on removing them first. I also have the feeling that you might not even have proper mechanical filtration - and I might be wrong - please clarify.
    – virolino
    Commented Dec 8, 2022 at 13:09
  • @virolino I updated the picture, hopefully that's better. Also, you are correct, there is a little bit of green algae in the tank right now, which the snails and mollies seem to be eating. There was brown algae for a little while, but the mollies ate it. I'm somewhat afraid to get rid of too much of the algae for that reason. Commented Dec 8, 2022 at 15:02
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    @EJoshuaS-StandwithUkraine too much nitrogen will kill your fish. Other fertilizers are safe. More research may also be necessary. Commented Dec 9, 2022 at 16:17

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From my POV, you are doing it the wrong way. One of the best fertilizers for aquarium plants is a cycled aquarium. The fish (and other "animals" you may have) make a lot of Nx substances, which are just perfect for the plants.

The most important thing to do is to get rid of all the algae. Even if the fish and the snails seem to eat it, they (the algae) create all sorts of mysterious problems there - one of them being lowering the level of oxygen in the water.

So please find some solutions on how to get rid of all the algae, use them (the solutions), stop fertilizing, and then see how things evolve. This path might require you to actually start everything over - which should not be excessively difficult, I do not see a lot of complicated details (e.g., landscapes) in your tank.

In my small tank, too much light allowed the plants to grow so much and so fast, that they killed almost all my fish and snails very quickly - with no fertilizer added at all. Now I keep things under control with a dimmer for the lights.

A small comment: I used some liquid fertilizer at the beginning, when there was pretty much nothing in the aquarium to resemble nutrients for plants. But that time was short, the fish created very quickly everything needed by the plants in terms of food.

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