The Fluval 403 is an old filter, from what I've read. Boasting a 317 GPH flow rate in it's prime. I suggest adding a secondary filter. When you add media, an inline heater, and inline CO2, you'll lose quite a bit of flow. I try to suggest at least 5x flow rate for a planted tank, more when you use CO2. Possibly consider adding a SunSun 302 or 304 if you are on a budget, or another canister of similar size as the Fluval 403.
Furthermore, I would be tempted to go with a CO2 reactor instead of atomizer for better CO2 use. This is opinionated, but a lot of people really feel strongly about a reactor.
T5s are fine IMO. Fine tuning your light window and CO2 will always be a challenge.
A Finnex aquarium heater controller is a solid investment. Definitely recommended with a Hydor.
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