I've known going in that the tank was overstocked. I just made sure to stay diligent with water changes and watch the NH3, NO2 and NO3. I used to have a wet/dry setup but I had NH3 and NO2 issues. With the floating media it pretty much fixed itself. I'll be moving very far away in about 6 months so most of this is academic for me to learn and test. I plan on a couple bigger more focused tanks in the new place. One solely for discus and the other for plants. I'll probably go back to a good canister or two for the planted tank.
Question on sealing the wet dry area, how would you go about maintaining good oxygenation for the bacteria if you seal it off? Is the oxygen in the water enough to be efficient?
I honestly thought what I had was at the least medium light. It's about 75w in LED's. 48V @ 1.6A. The whites are about 54W total.
It's interesting you've had that much difference between the w/d and sump filter. In that case, I wouldn't change a thing except sealing it IF you still want CO2.
Personally, if the plants are mostly anubias and ferns I wouldn't bother with CO2. They are both slow growers with and without CO2. Sure, they do better and grow faster with CO2 but honestly, not that much in my experience. On the other hand, poorly delivered CO2 can cause all plants to do poorly as well as killing inhabitants.
Sealed wet/dry filters have plenty of oxygen. In fact, it's the same amount that your water has. So yes, plenty of oxygen. Let's not forget the O2 the plants are producing.
Sealing the sump will trap all CO2 from escaping. That's pretty straightforward right?
Well, if you think about it you'll have a problem. You're pumping air into a now sealed chamber. That would clearly cause an issue. Instead, I would replace your air hose with a water hose. The tank water should have more than enough oxygen for the bacterial colony. Again, lets not forget our friends, the plants. So a strong power head churning the water will provide the bacteria with a constant supply of nutrients and oxygen.
As far as the lights, I think you may have misunderstood. I was saying you could probably do with LESS light. I have no idea how much PAR your lights produce. Typically, we don't need as much light as we use for good growth. If growth is what we want, CO2 packs a bigger punch. A great article about this is, CO2, light, and growth of aquatic plants.