Before you make the jump to Co2
are you prepared for weekly trimming?
That being said I wouldn't get an emtire system at once, it's nice to have the flexibility of building your system, for a 90g I'd reccomend a 5 or 10 pound tank instead of the really small ones you see with complete systems.
For the diffuser the Fluval 88gr ceramic diffuser is a good option, but inline (if possible) would be better. If you don't have a canister just place the diffuser under the intake.
For the BC the aquatek one is good for the price, but buy 2 if you order it ao you have a backup. The plastic is very brittle and can break easily.
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a 29 gallon high tech planted Chasing Red
, a 20 long low tech, and a 75 gallon salty adventure