Nitrate issues in 300g fowlr tank

Lion-o

Member
Hello!

I am battling a nitrate issue in my 300 gallon fish only tank. Nitrates are testing as high as my test kit will go (160ppm). I am not sure on my weekly growth as I haven't tested in a while, I admittedly got lazy with a happy tank. I am not new to the hobby - the tank is about 10 years old now and I haven't done a water change in a couple years. Nuisance algae died off and P04/N03 were doing good previously from vodka dosing alone. I do have some fuzzy purple looking algae (not sure on species), but not sure if that is an issue or not. I dose 19mL vodka a day (which seems like a lot to me). It wasn't until I lost a couple fish that I started doing a full panel and found this out. At that point I measured:

0 ammonia
0 nitrites
160+ppm nitrates
2.2 phosphates

So far what I have done is a 50gallon WC (16%). I blasted the rocks with a powerhead and did a gravel vac, and cleaned out the free debris in the sump.

Then I had a service come and do a ~160gallon water change (~50%), which cost me a pretty penny ($270). I tested a day later and nitrates still 160+ppm. I plan to test phosphates again, but I ran out of reagents for my hanna checker and need to order some more.

I have cut feedings down by half, plus have 2 less fish now producing nitrates.

I have an octa 205i skimz internal skimmer. Supposedly it's rated for up to 500gallons, so I believe it is already oversized for my tank. I do have a difficult time dialing it in to where it needs to be though.

I have a 40g reef tank in the same room that is thriving. I do 5gal weekly water changes there, all tests are good. It's just another beast entirely to do meaningful water changes on the 300gal. Looking for inspiration or advice.

I use RO/DI water only in both tanks.

I would like to do more massive water changes, but concerned with the cost and it possibly not solving my issue. I would also love to replace my sump, but it's barricaded in there pretty well by support beams so it would take some real surgery to get a new one in ($$). I might be able to cut two holes in the side walls, replace with a cabinet door, and then slide a brand new one with new plumbing. I wonder if I should reduce the vodka dosing or increase it. I have thought about buying an XPORT-NO3 brick (or 2) but I think it might be the same thing as the ceramic media I already have in the tank. Also read about a biodenitrator which sounds like it could help.

This weekend I plan to do another gravel vac (16% WC), do a full rinsing of my ceramic media, empty out my ATO reservoir and clean with vinegar (over time that thing always develops a thin slime, not sure what that is), and just clean out my dual carbon reactor for good measure. I will test again after all that and post an update.

I've also included some pics :)
 

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I prefer to drink vodka and dose vinegar.

I'm a scotch man myself. :spin3:

The main reason I dose vodka is because you have to dose much more vinegar to get the same results. I have no real preference one over the other, and I think the vodka might be cheaper in the long run.
 
I'm a scotch man myself. :spin3:

The main reason I dose vodka is because you have to dose much more vinegar to get the same results. I have no real preference one over the other, and I think the vodka might be cheaper in the long run.


Well been doing vinegar for years. My wife gets it at one of those wholesale clubs big bottle.


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Yea, nothing wrong with vinegar vs vodka or even a blend of both. I think they're all accomplishing the same thing in the end.
 
Yea, nothing wrong with vinegar vs vodka or even a blend of both. I think they're all accomplishing the same thing in the end.


So I can't help you. Not sure how 19ml vodka equates to 80ml vinegar.


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Fish only tanks can run high on nitrates. Just the nature of the beast
Oversized skimmers can be "finiky", working on & off because they pull their load & then idle waiting for things to build up again & the cycle repeats. If this is you you now know why. On my 250 I do water changes every week, 15 gallons, so it makes things easier @ only about an hour. 20 gallons a week would give you 80 gallons a month, not bad on a 300 with PO4 that high.
 
update, cleaned out the sump and ceramic media today. still making water to do a 50g change tomorrow and gravel vac. Not expecting it to make much of a dent in the nitrates though.

At this point, I'm leaning towards the sulfur reactor.
 

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Not sure why you vacuum I was always told that's a no. No. That could be the reason for the nitrates also.


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Not sure why you vacuum I was always told that's a no. No. That could be the reason for the nitrates also.

I think that is a possibility that vacuuming the sand could cause you to test higher nitrates, because you are stirring up the sand bed and all the "gunk" in there is being released into the water column. In my case, that is not the issue because the nitrates were already high.

Personally I believe vacuuming the sand bed is good for it. I do it weekly on my 40gal reef and it is a thriving tank. Unless you have an amazing clean up crew that stirs and covers every inch of your sand , gunk will pile up in there and on the surface. Many years ago, my algae used to grow right across the top of the sand bed. Best thing to do in that case is just remove it.
 
Man-made biological filtration media like your ceramic media are often considered 'nitrate factories' so you may want to reduce or eliminate that.

Vacuuming the sand is OK, but don't go deep. It disturbs the helpful bacteria you need badly. From the pics it looks like you may have done just that, so you need to leave it alone and let the bacteria recolonize and expand its population. All wet surfaces are coated with helpful bacteria. If you clean everything, you're eliminating a lot of bacteria. It'll come back with time.

Deep, fine-grain sand beds can help. They not only perform nitrification, but also denitrification, which lowers Nitrate.

Consider adding plants. They love Nitrate and will suck it right up for you.

While you wait for your bacteria to bounce back, do whatever water changes you can to dilute the Nitrate.

Hope this helps. Good luck!
 
Consider adding plants. They love Nitrate and will suck it right up for you.

I actually just added a chunk of codium yesterday (looks like that is your avatar :)) and my butterflies have been picking at it. Not sure how long it will last or if it will get overwhelmed by the nitrate levels.

Also ordered a sulfur denitrator, coming to me in chunks in the mail. I plan on setting it up tonight with tap water just to do a leak test.
 
A really good skimmer might also help. I dosed just about everything, and a skimmer improvement solved the whole issue. I no longer dose anything.
 
A really good skimmer might also help. I dosed just about everything, and a skimmer improvement solved the whole issue. I no longer dose anything.

Did you end up upsizing or downsizing your skimmer? I have heard recently that downsizing could actually produce better results.

I have this skimmer, which seems right for my 300gal. I thought it was a good skimmer when I bought it :)

https://www.bulkreefsupply.com/octa-sc205i-internal-protein-skimmer-open-box-skimz.html

I'm not positive as to whether or not a skimmer should be considered a key part of the N & P reduction program in your tank, but it is undeniably a crucial component in a sump for all the benefits it does provide.
 
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