ok...lots of questions in there...
Firstly, discus and bristlenose. normally ok, just watch out if the discus are breeding cos they sometimes like to suck the slime off the sides of the breeding discus.
you have good filtration
CO2 is a good fun thing to mess around with but first things first...its not cheap to do properly and i would not go any other way. dont expect to see any change out of about $600-700. and if you have the CO2 you wont need the airstone as they airstone will knock the co2 out of the water and it would be a waste.
UV is awesome. however, a 5watt unit is going to be next to useless. I use a 9 watt one on both my big tanks and its awesome!! it is a unit that emits UVc light and it kills everything, algae, fungus, bacteria, parasites...eveything. its awesome!!
i know lots more about that but its late and im tired and cbf writing more
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