Something I'm not sure has been touched on in any of your threads is, are you testing your water with a liquid test kit? I'd be curious to see what the nitrate reading is on the 10 gal with the plec that you haven't been doing water changes on, I'd suspect it's at dangerous levels. It's crucial to have one of these test kits to properly run an aquarium. There could be toxic levels of ammonia, nitrite, or nitrate present in your water that wouldn't even know about otherwise. If you don't already own one, I'd highly recommend you pick one up as soon as you can. It's definitely imperative you have one before starting your new tank.