Case of the disappearing Nitrates on new 55 gal tank setup

OK water chemistry / cycle experts. Perhaps you can shed some light on where my nitrates are disappearing to !

About 3 months ago I started setting up a new 55 gal. At the moment it's stocked with four 4-5" tinfoil barbs, two 4-5" parrot fish, and a 5" pleco.

Filter system #1 is a Penguin 330 HOB running only the two original filter cartridges (rinsed out several times) and the bio-wheels. The suction pipe is fairly close to the middle of the tank and in very close proximity to a bubble wand.

Filter system #2 is a Danner Mag 7 submerged at the far right end of the tank far from the bubble wand. This pumps to a Rainbow Lifegard AF-93 module with pleated paper filter. This is direct coupled to an AF-94 module with about 8 ozs of carbon. This is direct coupled to a QL-8 module with an 8 watt UV lamp. This is direct coupled to an AF-92 heater module with the heater removed, the top heater fitting closed off with a valve, and instead the module is filled with 1" bio-balls. From this Rainbow Lifegard module lineup, a return hose dumps the filtered water into the intake fitting of an el-cheapo AquaTech HOB running cut-to-fit 1 micron Rena filter pads as a post-filter (the AquaTech pump is not used). The actual flow rate through the Rainbow Lifegard module lineup is a bit over 200 gpm, which is about 1/2 the flow rate which the modules were designed for.

Because my parrot fish love to dig, and because my tinfoil barbs love to nibble, so far I have been unable to keep any plants in the tank long enough for them to 'take root' and grow.

I also have well water which is low in hardness but a bit high in phosphorus - which along with a sunny tank location was the motivation for getting the UV unit to help control algae (which it does very well - just a few algae stains on the bubble wand so far).

I have only been doing a 15 gal FWC on a weekly basis for the last month since the tank finished cycling.

My question is this. My ammonia is essentially zero. My nitrites are essentially zero. My nitrates only get up to about 10-15 ppm prior to the weekly 15 gal water change. Where are the nitrates going ?

I have a bizarre theory that I may have accidentally hit on something, but I don't even want to speculate until I hear what you experts think first.

Kasakato said:

So you have a 50g tank, with those 2 different filter systems...correct?

Click to expand...

Correct. The two filter systems operate independently and continuously. The Penguin 330 HOB draws water near a bubble wand close to the center of my tank, and the Mag 7 pump draws water from the bottom far end of my tank.

And you change 15g weeky?

Click to expand...

yes, 15 gal out of an actual estimated water volume of 45 gal = 33%

And what were they (nitrate) before and after?

Click to expand...

10-15ppm before ... 7-10ppm after ... a 33% drop more or less expected given the water change volume. I can't say this for sure, but it seems like both the before ppm and the after ppm are dropping slightly with each passing week even though my fish are growing visibly bigger. Based on the size of my fish in a 55 gal unplanted tank, I would have expected to have a difficult time holding the nitrate ppm below 40 doing two 15 gal water changes per week instead of just one !

Did you do anything different?

Click to expand...

Only removing the 200 watt heater from the Rainbow Lifegard module, filling the module with 1" bio-balls, and directly installing the heater immersed in the tank.

PS besides phosphorus, my well water also contains a bit of sulfur and some other minerals besides iron and calcium (which are low).

PPS the only 'surprise' I noted with this setup is that the 1 micron Rena filter pads seem to be collecting a very fine gray-black colored deposit on the upstream surface - I would guess that this is actually very fine carbon powder which is being leached out of the activated carbon chamber of my AF-94 chemical filter module and passing right through the UV module and the bio-ball module.

How offten do you clean the filters?

Click to expand...

I rinse the Penguin 330's filter pads and rinse the 1 micron filter pads in the AquaTech post-filter once a week in hot well water during my 15 gal water change. Every 3 weeks I rinse the Rainbow Lifegard's pleated filter cartridge, replace the carbon (which does slowly clog and causes the back pressure to rise), and wipe off the quartz glass surrounding the UV lamp. I haven't disturbed the bio-balls, but then again the bio-balls have only been in the former heater module for about 5-6 weeks now since I moved the heater directly into the tank to improve temperature regulation.

Do you still have plants in there?

Click to expand...

Well, I have plant 'bulbs' in there ... but every morning I find that the parrot fish have dug up the bulbs again, and that the tinfoil barbs have chewed off anything green trying to grow out of the bulbs LOL.

What else do you do in regards to maintenance?

Click to expand...

so far, absolutely nothing - not even vacuuming the gravel. I do lift up and wiggle around the Mag 7 pump every day to stir the gravel in that area, because any excess food seems to drift to that end of the tank and collect in the gravel around the pump. My tank water is crystal and I mean crystal clear ever since I swapped out the standard AquaTech filter cartridges for 1 micron Rena filter pads about 5-6 weeks ago.

Just a side note ,are those tonfoil barbs going into a bigger tank? If not I suggest you think about it, a 55g is no where big enough even for just one.

Click to expand...

Yes I know - a new 300 gal tank is on the agenda for next winter. Hopefully the tinfoil barb's growth rate will accomodate my new tank schedule !

my bulbs might be sprouting perhaps two or three green 'stalks' of maybe 1/4" in length before the barbs nibble them off - there are no real leaves or stems or roots present - thus I can't believe this tiny bit of sprouting greenery can be responsible for the large amounts of nitrates which are seemingly disappearing.

Madness said:

Hi, Melonie.

I only know of three ways to eliminate nitrate outside of water changes.

Anaerobic denitrification.
Plant consumption.
Chemical bonding.

Which do you think? Or do you think it is something else? You haven't mentioned the use of any chemical bonders and you seem adiment that it is not the plants. Do you think your filter chain is long and slow enough to denitrify?

Click to expand...

Well 'door #2' is doubtful, since there simply isn't enough greenery present. 'door #3' is a definite NO, since I haven't added any chemicals which could act as bonding agents. By process of elimination this makes 'door # 1' a likely suspect. I've been mulling over my situation and note the following ...

- The pump intake which feeds my filter modules is in a corner of the tank which doesn't get any aeration and which has lots of organic 'fallout'
- Each of my modules holds 1/2 gal of water, and flows from bottom to top with a submerged 'overflow pipe' (which gives air time to rise to the top in each module and then be trapped there)
- I'm running a pleated paper filter cartridge in the first module which, when removed for cleaning every 3 weeks, looks for all the world like a bio-wheel on steroids (same color thin slimy deposit as the nitrifying bacteria on a bio-wheel, but on a pleated surface about 10" long and 2 1/2" in diameter)
- I'm running 8oz of carbon in the 2nd module, which begins to clog up after 3 weeks or so because it's packed with something ... but based on my water change volume, well water clarity, and carbon volume, it has to be something other than absorbed minerals/chemicals (my guess is nitrifying bacteria)
- I'm flowing at a significantly slower rate past my UV lamp than it was designed for (actually I'm only flowing at about 50% of rating through all of the modules) thus the 1/2 gal of water in the UV module is getting some serious UV exposure
- A fine carbon powder appears on my 1 micron post-filter, meaning that tiny carbon particles are leaching out of my carbon module, and flowing on through my UV module and my bio-ball module

Given the above, my own weird theory is that by the time my water gets to the 4th module with the bio-balls in it there isn't much dissolved O2 left at all because each previous module has mechanically separated some air which rose to the top and was stuck there, plus the nitrifying bacteria in the first and second modules has used up much of the dissolved O2. Also an ample source of carbon is available i.e. the fine powder leaching from my 2nd module and being pumped all the way through the UV and bio-ball modules to my post-filter. From what I could research about denitirifying bacteria, absence of O2 + a carbon source are the two major prerequisites. Also I know very little about this aspect, but some basic atmospheric UV chemistry research shows that my UV module might possibly be converting any remaining dissolved O2 first into Ozone and then into Nitrous Oxide gas, which would tie up some N2 by itself plus leave essentially zero dissolved oxygen in the water by the time it reaches the 4th module with the bio-balls (and my fish do act extraordinarily happy all the time LOL !).

I'm thinking that my bio-balls in the 4th module are actually supporting anaerobic denitrifying bacteria on a fairly large scale. Either that, or my UV module is causing a non-bacterial denitrification via O2+UV --> Ozone+N2 --> Nitrous Oxide gas conversion. Or maybe both ! Or maybe I've just been breathing too much Nitrous Oxide myself !

However, if there is actually a UV driven non-biological denitrification process going on, it might open the door to some very interesting future filtration equipment possibilities.
~