it's all about the strength of the bulb and the detention time in front of that bulb.
For simple phages such as single celled algae, or bacteria, a low watt unit will work fine.
For more complicated protozoa with (comparatively) hard cell walls, you will need a strong enough wattage unit, and slow enough flow (detention time) to keep these animals in front of the bulb long enough to scramble their organelles. If flow is too strong, they can pass thru the unit unharmed.
For ick, you must also take into account getting the stages of life where the protozoa is planktonic. while on the fish of course, the UV has "no" effect whatsoever, or when ick is in a dormant stage in the substrate, again, probably no effect (unless they are stirred up and sucked up by flow).
Even with a UV in a 210 gal tank you will probably need to treat the tank with salt, or some type med, because it could take months to get every individual parasite to flow thru the UV unit long enough to kill all.
Once eliminated, the UV could help, but a quarantine tank would be much more practical.
UV bulbs need to be regularly replaced because they lose intensity, and the tube housing needs to be kept free of debris or the will be ineffective (maintenance).
