UV Sterilizer and oxygen levels

laga77

New member
Picked up a flame angel and golden headed sleeper three days ago. They are in a 30 gal QT with a sand bottom and some cycled rock. There is a HOB filter with carbon and floss and a small HOB protein skimmer. After the first night I noticed a few white spots on the flame. I hooked up a 55watt UV sterilizer from Amazon and a diatom filter. After about two hours I checked on the fish and they were in distress. Heavy breathing and laying on the bottom. My only guess at the time was the impeller of the diatom filter had somehow effected the gas levels of the water since I had a ball valve at the discharge to slow water flow. After shutting off the diatom and sterilizer I pulled water from the tank and poured between two buckets and back into the tank about twenty times. Luckily, I had guessed right as the fish recovered. Today I tried just the sterilizer with a submersible Rio pump rated at 1500 LPH, no ball valve. After an hour they showed distress and I immediately stopped the UV and the fish are fine. So, it was not the diatom, it was the UV creating the problem. The temperature of the water did not change with the UV. Now, I have an identical UV running for the past 6 weeks on my 120 gal reef with no problems and I run the diatom occasionally on the 120 and a 50 gal reef as a clarifier and water polisher as needed. Like after a major cleaning. I am at a complete lost for an explanation. I cannot find any info about UV effecting oxygen levels, only about UV causing formation of ozone. Any ideas?
 

Timfish

Timfish
Premium Member
Very interesting problem. I've been using UVs for decades on my QT and some of my display tanks but haven't seen a problem like that. I also have very little experience with ozone but I would expect the fish to show distress if the UV was generating a fair amount but I want to emphasize that is conjecture on my part. Did you by any chance check any other parameters besides temperature before, during or after?
 

laga77

New member
The only parameters checked were temp, PH, SG and alk. 79F. 8.0 1.026 and 8.7 dkh. They have stayed stable.
 

slief

RC Sponsor
Premium Member
My first guess would have been something to do with your diatom filter but it seems you have eliminated that possibility. In over 20 years of running UV filters on multiple tanks, I've never seen any issues related to depleted o2 levels. In fact, because it oxidizes organics passing through the UV, I would expect increased o2 levels after a short period of running. I've actually seen increased ORP as a result of running a UV. Perhaps your water has so much organic in it that the stuff killed or oxidized by the UV is causing a low o2 situation to get worse or is effecting the fishes gills. Point being that if the UV is causing a hypoxic condition, your o2 levels are already perilously low. Perhaps you should fire that UV back up and add an air stone to the tank near a power head. A HOB skimmer on a 30g tank certainly isn't going to do much for saturated o2. You will need a lot of surface agitation and or a decent overflow to augment o2 levels under normal circumstances which it sounds like you may be lacking.
 

laga77

New member
Besides the HOB filter there is a Coralife Biocube protein skimmer inside the tank with the air flow turned full and a powerhead. So I believe there is enough aeration for the tank. I believe you are correct about the dissolved organics causing the problem, along with the UV being too large for the tank. After doing more reading last night this seems to be the logical conclusion. Where all the organics came from is the next mystery because the tank has a very low bio-load with regular water changes and maintenance. Fish are doing fine this morning. The flame has two specks and is eating good. Will do TTM to rid the ich. Thanks
 

laga77

New member
Is there any chance you can hook an orp meter on that tank for a couple of weeks

I don`t have one nor do I know of someone with one.
The fish are doing fine in the middle of their TTM. I tore the tank down to clean it out and will set it back up as an observation/quarantine tank. It sounds like an interesting experiment. Maybe I will start looking for an inexpensive ORP meter.
 
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