Hi and welcome to the forum
Have you checked your tap water for nitrates?
If you have high nitrates in the tank water, then it will keep the high level in the tank.
If the tap water has no nitrates, then you should do bigger water changes more often. I recommend doing a 75% water change and gravel cleaning the substrate once a week. If your tap water has no nitrates and a weekly water change does not reduce the nitrates, then do it 2 times a week.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Established filters should be cleaned at least once a month and preferably ever y 2 weeks. Wash filter materials in a bucket of tank water.
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Anubias is useless for absorbing nutrients out of the water. It is a marsh plant that grows very slowly under water and as such it absorbs virtually nothing from the water.
Water Sprite (Ceratopteris thalictroides/ cornuta) and other floating plants like Duckweed will grow much faster ad use a lot more nutrients (nitrates) than plants growing in the gravel. If you get too much Water Sprite, you can plant some of it in the gravel where it will grow into a nice light green shrub.
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The fish in the pictures appear to have a bacterial infection. This might have been introduced with the eel or it might be from too much gunk building up in the tank, gravel and filter and the eel and frozen bloodworms made the problem worse.
Wipe the inside of the glass and clean the filter if it hasn't been done in the last 2 weeks. Then do a 75% water change and gravel clean the substrate each day for a week. See how the fish look after that.
If you can post pictures of the fish in the tank, I can check them for spots or marks that might indicate other health issues.
If any more fish die, post a pic asap and we will look into a medication for the tank. However, try big daily water changes for a week and see how they look after that.