Thanks for some really good replies! I've got too much invested to quit! LOL! OK, some answers/replies...
1. No quarantine as I do not have a tank flourishing yet
2. I've not put more than 2-4 small fish (small Comets + Otos) in either tank + a couple snails at any given time, trying to manage the bio load at least somewhat. I wouldn't think I'm overwhelming the tanks, but it certainly is possible.
3. 10 gallon runs clear but no algae, it's moderately to well planted, I fertilize once a week. The 20L runs a little bit cloudy, outbreaks green water sometimes, and has no problem growing algae. Parameters have been really good for a couple weeks now, just Nitrate running high (one plant in this tank, no ferts). I clean up the green water by cutting off lights for a day or two.
4. The fish in local Big Box Pet Store, Inc are kept in less than ideal conditions, goldfish crammed in, Otos look skinny most of the time, so I think I will change up to Tetras or Danios...oddly enough, they usually look in pretty decent shape when they have them and the tanks have been maintained. One of our stores has some pretty good fish people that seem to care. The other, not so much, but I'm still reluctant to make major fish purchases from either.
5. My tap water is town water, so it's inconsistent as it can be (especially pH and Ammonia). Before putting it in the tank, I filter it at the spigot, use Prime/Water Conditioner and let it set at least 24 hours before use.