My thoughts/answers inline in red....
Hmmm... I'll throw some thoughts your way...
Is there any reason why you keep your tank at 80 degrees?
Yes, there is. I prefer cooler water tanks, but I really like the gouramis, and 80 degrees is at the low end of what the gouramis are comfortable with.
Your lighting isn't high, and I see you are not dosing N, P, and K... this should be fine. Is there any chance your nitrate is coming from your fert tabs? I'm not sure what is in them... do you disturb your substrate a lot?
This is an interesting point. Come to think of it, I had been using Seachem Root Tabs, but ran out and didn't realize I was out until I went to use them again just recently. So, I used some other root tabs I had on hand that I didn't like as well. I do not know what ingredients are in these alternate root tabs.
As for disturbing the substrate... yes, I do. Very few of my plants are root feeders (namely just the vals and the amazon sword). So, to keep the gravel in the rest of the tank clean, I do vacuum the gravel. I do my best to stay away from the areas that I've put root tabs in though. Could this be part of my problem?
Have you calibrated your test kit? Tested your tap water?
To my knowledge, there is no way to calibrate the API test kit. If I'm wrong on this, please let me know.
I just tested the tap water. I got distracted and let it sit for 10 minutes instead of 5, but the tap water registered at between 10-20ppm. What is normal for tap water? Is this reading high?
I agree with the others who have suggested one large weekly water change is likely to be more beneficial to several smaller ones throughout the week (not meaning to belabor the point)...
This is my preference, but from what I've read a 20% weekly water change is usually sufficient. In my case, it is not. So I increased the number of water changes per week. I will do a large water change today and then keep an eye on the levels throughout this coming week and see what happens. Thanks for the input!
I agree with racer... clean off as much as you can, then do large WC, and make sure your filter is clean (of course you know to preserve your biofilter... I'm just including that for others).
Saturdays are the day I prefer to do water changes, so I will do that today and see how it goes this coming week.
I have a limb from a hedge tree in the tank. Could that be causing problems? Hedge is a very hard wood and is often used for fence posts here.
Also, I have a 3g bucket full of trident java fern that I have been considering adding to this tank. However, I would rather sell the java fern and buy a different plant(s) if the trident java fern is only going to melt away and die in this tank. I had it in my 29g tank that I am taking down, and it did fine in there. Thoughts/suggestions?