re: UV Filter options 2- 18watt or 2- 36 watt

I would not use most UV uniys inline with a canister. The odds of achieving the desired flow rate are not that great. Morover, you are then married to that configuration. I think it is better to run a UV unit on its own independent power head or pump. One other suggestion. Buy an extra bulb when you buy the unit. better to have it on the shelf than have to scramble for one down the road.

I use Hydor inline heaters- I have one of each wattage and they have served me faithfully for almost a decade now. I even have one plumbed on the intake side of the canister as there was not enough space to do it on the output end where it should normally go.
What makes the common man uncommon is common sense.

Re: UV Filter options 2- 18watt or 2- 36 watt

Given time, UV radiation will degrade the phyto components and the water will clear up. This said, some extracts seem to be more durable than others. Also, there is concentration playing a role as well as UV rating. The stronger the concentration, the more time it will hold up in the water column because it's harder for the radiation to penetrate the passing flow volume. Also It will depend on how you administer the dose, if all at once or continuously, that is, a specific measure vs extended release. So you see there are several factors that will affect the performance of a black water extract in a UV radiated water column. Exposure time, UV rating, water volume and concentration, all play a part. The commercial products will kill your wallet...well, at least mine.
Translation. The darker your water, the less effective your UV.
God listens. He may not always give you everything you want, when you want it; but he always gives you what you really need, when you need it most!