How does your flow work in your tank? Do you have a tank journal? You may want to move your koralia power head closer to the surface, or add a second power head in to oxygenate the water more.
Keep up with your water changes, and maybe look into some sponges, like a nitrate sponge, as something to keep on hand for emergencies.
How about some photos of your cylinder tank? I'm really curious as to what it looks like?
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