Quick Question - UV Light Questions - Need to order overnight asap

[WhatCouldGoWrong71]

I am putting together a system with a 55G display and a 10G sump this weekend and want to get a UV light to plumb in. Should I get the 18 which is 10% undersized or the 25 which is much larger. I am not worried about the $50 or size difference. But concerned about being fine at 10% undersized or or not good with so much overkill? Thoughts?

Pentair - Smart UV Sterilizer 18 Watt (Up to 60g) or​

Pentair - Smart UV Sterilizer 25 Watt (Up to 130g)​

 

[SPS2020]

I have a 40w IceCap on my 60g and a 57w on my 100g. I'm a fan of overkill.

 

[dadnjesse]

I would go oversize.

 

[Tuna Melt]

It depends on your objective, are you trying to combat Ich, prevent algae blooms, or just looking for clear water? When I looked into UVs a month or so ago to combat Ich, one thing was clear, bigger is better. Its kinda an arcane topic without a ton of clear guidance but this is what I learned when I was researching it:

There is a minimum UV dose required to combat Ich thus a minimum UV size to deliver that dose. Conventional wisdom is 280,000 uW/s/cm2 kills the theronts. Per this table from Pentair (kinda the Cadillac of aquarium UV manufacturers) their 18w and 25w unit can't actually hit that dose, and thus are not effective against Ich:

That being said, this article (https://www.reef2reef.com/threads/finally-cryptocaryon-uv-dosage-data.892710/) claims a dose of 69,000 uW/s/cm2 is capable of reducing parasite abundance per fish by 97%. Randy Holmes-Farley pointed out that we don't know how absorbent our water is so we cant really be sure of the actual UV dose realized in our aquariums. If you have brown murkey water the UV dose will be less than if you have crystal clear water.

Just some food for thought.