That is crafted so questionably, that it is either horribly in error, or deliberately meant to be disinformation.
Saying the above, is equivalent to saying, "A filter can only treat the water flowing through it."
I mean, duh! However, after only a part of a day, I would expect all the water in the aquarium has gone though the filter. If not, its got the rest of the day, tomorrow, the next day, etc.
So does UV work. Just as well does UV work (depending on flow rates through UV unit--just EXACTLY like the filter on your tank.) In other threads, I have mentioned flow rates I would recommend--to state an acceptable rule of thumb--1/2 watt per gallon UV with a flow rate AT LEAST 1/2 tank volume per hour--tank volume per hour is good also. At that rate, the latter, tank volume will go through the UV unit 24 times a day!
Any quality fish store runs UV. Even walmart runs UV.
If someone tells you that UV is BOGUS or NOT IMPORTANT, they are DEAD WRONG. Are they just ignorant to the benefits? Are they attempting to "hold an upper hand" by seeing you not run it and they do? I don't know ... makes ya' wonder after you run UV though!
Here is but one page on UV:
http://americanaquariumproducts.com/AquariumUVSterilization.html
there are many available on the web.
Remember, mother nature uses UV in all her ponds, lakes, streams, rivers and oceans--mother nature is never wrong!
Regards,
TA
