It looks really nice. I have a few questions if you don't mind:
- First of all, I'd like to know if you have already found out a fertilization routine.
- Second, I'd use a co2 atomizer instead of that ceramic diffuser, you will see changes in growing for sure. In case you don't, make sure the in-pipe helps in moving the co2 bubbles, like placing the diffuser in the opposite corner of the out pipe. In your case, exchanging the drop checker and the diffuser will make it work better disolving the bubbles into the water, I guess, and also the same accurate measure of the co2 concentration with the drop checker.
But as I said, much better with an atomizer. I can tell you where I bought mine if you wish, is a high quality one and not expensive at all.