Nitrate levels in a tank

[jonv]

While I was cleaning out the tanks tonight, I started thinking about the nitrate build up in the tanks. I was a little slower then usual when cleaning out the 100 gallon and ended up going 10 days for the change instead of the usual 5-6. So I figure my nitrate levels must have been higher.

Let's say for example, if you have a tank that's running like 80-90 in your nitrates, now you want to begin doing a water change. If you have a larger type tank, something like say a 150 or higher, you're talking about larger volumes of water. Now I'm pretty sure the distribution of nitrate in tank water is pretty even, but does anyone know if this is really true? Are your nitrates basically evenly distributed in your tank water?

What this led me to ponder was, changes in my tanks do take time when I do them, because I do have to use the bucket method when refilling. I've been doing it this way a couple years now and kind of used to it. But, let's say I've first drained the tank now, 50%. This takes something like 20-30 minutes while I vacuum out the gravel well too. While that water is going down, you're lowering the water with and without nitrate in it. Assuming there is an even distribution, there shouldn't be any increase or decrease in your nitrate ppm readings, if you have say 10 gallons in there, 20 30 or even 100 right?

My ponder is that is it possible that you could temporarily increase your nitrate ppm when doing a change? Is it possible that taking the water out to change it might take out water that does not have nitrate and thus in effect temporarily increase your nitrate ppm? In my 180, it takes close to an hour to refill what I took out so I just wondered, if nitrate ppm could have a temporary increased reading due to water out of the tank during changes, couldn't this theoretically as well, bring your nitrates up to a very lethal level?

[bikeguy33]

great question. carl may be able to elaborate more....but speaking my own lil bit of logic.....and not the gospel truth....i`d think that levels are higher at the bottom of your tank....since that is where 90% of your tank material is. even through vacuuming, there will still be nitrates left i`d think. carl has pointed out that water carries very little as for bacteria...so i`d have to assume most of the bad stuff is attached to substrate

[Carl]

A good and interesting question

Generally speaking the nitrates are quite evenly distributed in the water column and I have never observed a reading or read otherwise.

Using a 100 gallon aquarium
In a pure water change (no vacuuming), if your nitrates were 50 ppm and you changed 25 gallons, your nitrates should be about 35-40 ppm.

However the gravel is another matter
This is where it gets more subjective, however I have again both found that in a well circulated aquarium, nitrates will "percolate" into the gravel and even fine sand quite readily to a depth of 2.5 inches. Past this depth, some mixing whether by fish, snails, and especially plant roots is necessary for better nitrate penetration.
Without this, instead of de-nitrification you end up with sulfur reduction and the production of hydrogen sulfides, especially in tanks with sand.

Now bringing vacuuming into the equation, theoretically you are likely to have more nitrates trapped in the first 2.5 inches of gravel that could be released into the water during a water change, however in tanks with deep sand, this would be offset with more nitrates penetrating deeper which would then off set any nitrates that may be released into the general water column.

I have made more observations of this in my marine tanks that backs up these theories.
For instance in marine tanks with deep sand beds over 5 inches (which is much more common in marine keeping to have this deep sand bed vs. FW), I have recorded nitrate levels before and immediately after a water change, followed by another reading a day later (no feedings in this time period). The readings before and after reflected the pure water changes of a 25% reduction (assuming a 25% change), HOWEVER the next day I noted LOWER yet nitrate levels showing that the de-nitrification process was given a "kick in the pants" by the vacuuming of the gravel/sand (my hypothesis is that both the reduction in "fuel" via organics was achieved but more importantly the nitrates were able to permeate the deeper layers of sand and be reduced)

It is funny that you asked this question as I am working on an update for my aquarium cleaning article to reflect similar questions.

I also have some related information in my new Hydrogen Sulfide article in Aquarium Answers

Carl

[8 in the Corner]

I read somewhere a while back that all the "big three" chemicals (ammonia, nitrites and nitrates) are concentrated a little bit more at the lower levels of your tank. Therefore, it recommended that when you siphon you should take the water from about 2" above the substrate. I can't back this up with the info on where I read it, but it stuck with me over the years.
John

[eve]

even without reading anything or looking up
i would think that it's really concentrated at the bottom where all the waste is building up

when i do a waterchange, i generally always suck everything out from the bottom of the tank, not in the middle or the top of the tank
but that's just how i always did it, so i wouldn't know more about it

just more a logic thing IMO

[demfish]

So when we do our water test, should we take water out from around the bottom of the tank?

[Carl]

Nov 24, 2008 19:43:25 GMT -5 demfish said:

So when we do our water test, should we take water out from around the bottom of the tank?

I honestly do not think that this would make too much difference, however it it might be worth while to check this (although if your circulation is good you likely will see no discernible difference).

I have done this using a piece of air line tubing to siphon water right from the bottom into a test water bottle.

Carl