Re. the tannins, from my experience only water changes will help. You change it, they leach out, you change it they leech out. If you don't change it, they still leech out and make this tea like tank water, then the light doesn't pass through as well, and then there are more problems. The problem will pass in time.
Nice to see you starting up another one. A bit surprised about the CO2 though. You do such an awesome job on your other tanks without it! Guaranteed my next tank (you know, the legendary Mini-M) will not have it. Trying to simplify things a bit..... or a lot.
Please keep the updates coming!