A UV or diatom filter will filter out the free swimming stage of parasites, worms, harmful free floating bacteria, etc. Basically, it eliminates everything (both good & bad) in the water column.
However, it’s exact effectiveness is determined by a number of factors: How bad of an infestation you have in your tank i.e. how many free swimmers are active in the water, which pathogen are you trying to manage (e.g. velvet dinospores are more numerous than ich theronts), the size of the UV/diatom being used in relation to the water volume of your aquarium...
In the end, I feel it is unrealistic to expect
100% eradication from a UV or diatom filter. A few free swimmers are always going to make their way onto a fish before they can be filtered out. However, the following greatly increases your chances of successfully managing disease via a UV or diatom filter:
- The larger the tank, the more diluted the parasites will be, the more likely they will not come into contact with a fish in lethal concentrations.
- Buy the biggest UV/diatom you can. Bigger is better here and you want as many GPH as possible, because an oversized UV/diatom will more effectively/more quickly eliminate pathogens from the water.
- Have a backup UV/diatom, because the moment your primary unit breaks your shield goes down.
- When running a UV, change out the lamp every 6 months for maximum effectiveness and make sure GPH is slow enough to effectively zap parasites. Most UVs have different flow rates for removing algae spores and parasites (slower).
- The small diatom filter I use (this one) for water clarity purposes clogs after just a few days. So, I take it apart, rinse everything out and recharge with new diatomaceous earth. I can do this whenever I have time because I am not depending upon it for disease management purposes. However, if that was it’s primary function then you would want a backup on standby or clean it/get it back online ASAP so there wasn’t much “down time” for removing parasites.
- You are far more likely to be successful managing ich or preventing a bacterial infection from spreading with this approach. IMO/E; velvet dinospores are usually too numerous to be effectively managed. Brook, uronema and worm infestations all have a direct lifecycle i.e. they never leave the fish, so running a UV/diatom might only protect other fish from getting the disease.