The 57 w or the 40 w will both work. You will need to adjust for proper flow for either to cover algae/bacteria and pathogens (when/if they ever show up in your tank).
You could drop down to the 40 Watt from AquaUV for example. For 180 gallon tank you'll need about 1440 gph through that UV filter to hit 8x turnover and still have 30,000 uw/cm2 exposure to damage/kill bacteria/algae. While the bacteria bloom you're experiencing can likely be controlled at much less flow, some algae/bacteria can have reproduction rates that can hit double in 20 min (some types of dinos) which is why the 8x turnover is best. No sense getting a UV filter unless you can fight the worst that you might see. Those filters are too expensive and getting one that's' properly sized for worst case will do more for you in long run.
If you ever had an ich outbreak then you'll need to drop the flow to 967 gph to achieve the higher exposure to kill pathogens. Luckily they have a lower reproduction rate.
No need to go oversized either really unless you can get one flow to cover both flow rates - which for 180 gallon is going to be silly expensive for the 160 w model.
If you want to stick the 57 watt then the flows would still be 1440 gph for the 8x turnover, but the lower flow would be a bit higher at 1066 if that works out better for you pump/design to get flow through UV.