You'll get a different answer from everyone answering this post, but to maintain stable water conditions, you need to flush a lot of pure, treated tap water through the tank every week. Your tank is really an "unflushed toilet" and by removing and replacing half the tank water weekly, you prevent nitrogens from building up in the tank. Some may disagree with such an aggressive water change routine, but the bottom line is "water changes work" and are the most important step toward a stable water chemistry, regardless of fish load.
This is how nature does it. The fish and plants get a constant supply of new, pure water and the old water is removed.
I would recommend addressing the water chemistry first and proceed from here. Start a sound water change routine and follow it religiously.
Just a thought.
I have been doing ~30% water changes weekly, but this results in algae growth, dying java fern (which I had previously thought was impossible to kill), high nitrates, and problems with ich and, occasionally, fin rot. The problems seem to go away if I clean the tank more often (with no other treatment).