Sizing UV Sterilizers

Variant · Apr 3, 2019

I am thinking about getting a UV sterilizer and wanted to know how oversized it should be to be effective. After reading numerous threads proper sizing seems to be important to ensure you get the benefits of UV.

So for example, UV Aqua has an 8 watt sterilizer rated for up to 70 gallons for saltwater. Is this good enough for my tank that has a ~38 gallon display (displacing quite a bit due to live rock) and ~10 gallon of sump water?

Or should I go for the 25 watt version that is rated for near 150 gallons? I want to oversized but when does it become too much?

kchristensen8064 · Apr 3, 2019

Following, I've been wondering this same thing myself...

jent · Apr 3, 2019

What's your goals with uv? If you just want water clarity improvements and algae, then 7w is fine for your setup. If you want to mitigate parasites, then 25 is probably better. In either case you need to make sure your flow rate matches your uv size + goal.

LobsterOfJustice · Apr 3, 2019

To me, the gallon size ratings make little difference. It’s just about flow rate. I think the most efficient thing to do is find a quality unit which has a recommended flow rate that is similar to that of your return pump, and then plumb it into your return line.

SPR1968 · Apr 3, 2019

It’s all about getting the correct water flow rate for what your trying to achieve. The more powerful the better really but as they get more powerful they also get much bigger in size.

I think UV-C is possibly more popular in Europe and I’ve certainly used one since I started out

From what I’ve studied it’s all about contact time and if you want to kill of certain parasites, like for example ich, you have to achieve 72000 micro watts/sec and there is a calculation you can do based on the UVC power and the flow rate to achieve this.

The UVC only kills things that pass through it, so of course it’s not 100% effective but it certainly helps, and in reality I can’t see any down side to using one other than changing the bulb every 12 months.

The following is a quote from DD (David Saxby’s company) which I found helpful when researching:

‘’For water clarity we suggest a U.V intensity of 30,000 microwatt's/sec/sq.cm and our U.V sterilisers are designed to deliver this at the quoted flow/contact rate through the unit.
The mistake that many hobbyists make with our and other manufacturers U.V systems is not getting the flow rate right through the UV and connecting any old pump to it.If flow is increased the bacteria/algae or parasites do not get the correct U.V exposure and many happily pass right through the unit.

This is of some interest when trying to kill the white spot parasite as a greater U.V intensity of 72,000 microwatt's/sec/sq.cm is required to kill it. So with this in mind we have to take the standard flow rate of the unit for 30,000 microwatts and divide it by 2.4 to give you a flow rate for 72,000 microwatts exposure time.

So you will need to divide the suggest 500l/hr flow rate by 2.4 giving a new flow rate through the unit of 208 l/hr to kill the white spot. This will give a 0.6 times turn over for a 350l aquarium so it maybe worth going for the 20 watt unit and running it at 416 l/hr giving you over 1x tank turn over an hour but giving the correct kill intensity for white spot.’’

Variant · Apr 3, 2019

So it seems like the 8 watt would work as long as the flow rate is good. Maybe keep the flow rate at the minimum requirement (to keep the bulb cool).

This way the water is clear and bacteria and water bourne parasites are eliminated

Looks like uvaqua has it broken down well. Flow rate to help eradicate some of the protozoans.

ca1ore · Apr 3, 2019

Probably the most coherent treatise on the subject. Any site that has some variation of 'will destroy the planktonic food supply for the reef tank' is immediately suspect in my eye as they've clearly never run a reef tank LOL.

http://www.americanaquariumproducts.com/TMCUVSterilizer.html

jent · Apr 3, 2019

ca1ore said:
...Any site that has some variation of 'will destroy the planktonic food supply for the reef tank' is immediately suspect in my eye...

I am curious to hear your thoughts here. This is something I have wondered myself, it definitely makes the water clearer. It seems to me like phytoplankton would be particularly at risk from a UV.

I still run a UV myself, but I also feed phyto to try and supplement the populations I assume I am devastating.

robert · Apr 3, 2019

I would suggest the largest you can get, but don't have unrealistic expectations...Its a tool and much depends on how you use it. Its going to have little effect on ich for instance - even if your flow and power are sufficient - if you draw and return from your sump as that's not were you find ich in the system. Ich stays in close proximity to the sand bed and unless you draw into the UV from the lower 5" of your tank you'll get little effect with respect to ich/velvet.

I use it as a general anti-bacterial anti-viral - as well as to denature/oxidize organics - improve clarity somewhat. Its not going to solve any obvious problems that can't me more easily solved by other means, but I see it as one more insurance policy with respect to that part of your system that you cannot directly observe.

It helps provide dilution - provides breaks against blooms - which in all cases buys you time to respond when the unseen aspects of your system begin to go out of balance.

ca1ore · Apr 3, 2019

jent said:
I am curious to hear your thoughts here. This is something I have wondered myself, it definitely makes the water clearer. It seems to me like phytoplankton would be particularly at risk from a UV.

I still run a UV myself, but I also feed phyto to try and supplement the populations I assume I am devastating.

An UV sized to affect free swimming ich, as example, will absolutely affect any free swimming plankton. The flaw in the stated argument is that the vast majority of reef tanks have no sustainable free floating/swimming plankton population; thus, the objection is absurd. Many tanks will have sustainable populations of copepods, or even mysid shrimp; but these are all benthic organisms. While a small percentage of them might find their way through the UV, the vast majority do not because they are not water column swimmers. If you actually do have sustainable populations of phyto/zooplankton, then first of all CONGRATULATIONS (and how did you do it?). Secondly, bad news - your skimmer and mechanical filtration are pulling it out anyhow.