FYI - Manufacturers of aquarium products can, will and do exaggerate
for easy examples read recommendations on heater or filter size per aquarium size
　
Ive been using UV lights for quite a few years and for 4 years of that Ive been raising/breeding Blue Dempseys which are well known to be prone to parasitic illness.
　
I use a formula of:
1W per 10 US Gallon of tank volume
10 gph per 1W of (actual) flow rate through the UV
Replace the bulb once per year (Jan 1 at my house)
　
Flow Rate x Wattage = Deathrate is a very good simple view point
but additionally...
　
Contact time - this is the amount of time the parasite is exposed to UV rays. Lower flow rates allow for longer contact times. Longer tubes might increase contact time.
　
UVs water volume - The more volume a UV light holds, the longer the contact time of the unit at a given gph. A longer unit with a low volume will not yield a longer contact time.
　
Distance from Bulb - This is a detail I hear very little mention of. UV rays dwindle very very quickly when passing through water. Thus a parasite that is 2 from a bulb may be virtually unaffected when a parasite .5 from the bulb is killed. So a fat UV unit will be less effective at a given contact time.
　
Prefilter - UV lights come standard with a quartz sleeve. This sleeve is scratch resistant but not scratch proof. Small particles can cause a hazing on the sleeve which will drastically inhibit the UV rays. Also particles will cast little shadows within the UV unit. If you are pushing the max volume through the UV light to kill parasites, these little shadows can be enough to allow the parasite to survive.
　
Bulbs - Bulb quality can and does make a difference also. The UV-C emitting quality is given to bulbs by including a gas and possibly a powder to the bulb during manufacturing. Naturally the volume/purity/quality of the additives will impact both bulb potency and life of the bulb. Another point to consider, the UV quality of the bulbs does not stop abruptly. It will slowly dwindle every minute the bulb is on. Thus on day 1 it will be far more effective than on day 200. The more you maximize your flow rate the more frequently you will have to replace the bulb to remain effective against parasites.
Continuous -vs.- Intermittent Use - Since the UV quality dwindles with use, running it intermittently is a logical consideration. But it should be known that the additives dwindle incredibly fast at the moment of ignition. It has been suggested to me that 12 on /12 off will be slightly lengthen the life of a bulb, but 8 on / 8 off will shorten the life span of a bulb. This information was the conclusion between myself a 30+ year union electrician (my father) and an electrical engineer (my uncle).
　
　
Again, be leery of what manufacturers tell you. I am not suggesting every word out of their mouth is a lie, but I am suggesting they will stretch the truth to make their product sound superior to their competitors. For example, if they find one species of parasite that is easily killed by UV-C rays, they may use this parasite in their tests. This could allow them to honestly say their light is effective against parasites at a given rate
when in reality it is only effective against this single species at that rate
　
My suggestion is error on the side of safety
dont push your unit
