40% every 10 days shouldn't be no issue thou I do a 30% w/c every 2nd week on my 55g and have no issue. Something is out of balance there to make it regrow like it did. Do you by any chance have NO's / Ammonia readings from after the black out vs. now? Cause that could be a issue right there and saying the Oto passed is the more reason to check there cause they're quite sensitive fish on NO's.
Also real carefully evaluate your feeding schedule; maybe go for 2x week no food at all to lessen this some.
The amount fish (co2) is not an issues in itself. This gotta be balanced with your lights which if I recall correctly it was in your case. These 2 then gotta be well balanced with the nutrition for your plants - Which brings the next question how's your plant growth? Here's 2 picture diary's of my tanks where you can see the growth development by dates http://www.tropicalfishkeeping.com/f...ary-55g-39001/
That should help you some to see how well or not your plants grow.