Cycling question...

[Lukara]

Hi, I'm fishless cycling a 50 gallon tank. I've been monitoring the ammonia and nitrites for a while now and something unexpected is going on.

I've been maintaining an ammonia reading of 2.4. The tank is showing the capacity to lower the ammonia to between 0.6 - 1.2 after a 24 hour period. Last week I tested my nitrites and they were off the charts and have been way off the charts for several weeks. I tested my nitrates and they were around 5. This is normal, so far so good. Last night I checked my levels again and still, the tank is lowering the ammonia to between 0.6 - 1.2. The unexpected thing is that my nitrites lowered to just 0.3 and my nitrates went up to 20 ppm. I double checked my readings with 2 seperate sets of test kits and the results were the same.

Isn't my nitrifying bacteria supposed to grow enough to lower the ammonia to 0 before my denitrifying bacteria get as far as they've come?

Thanks in advance for your input.

(Edit) Additional information:
Tapwater PH: 7.6
Aquarium water PH: 7.0 (taken at last night's test)
KH: 20
Plants: None
Temp: 84 degrees
Filters: Aquaclear 500 + Fluval 304

[Lukara]

Anyone?

[JSchmidt]

Actually, it looks like everything is going OK. You've got a decent population of ammonia eaters and the nitrite eaters seem to be establishing themselves. It shouldn't be too long before you're registering zero ammonia and nitrites 24 hours after dosing the tank with ammonia.

One thing to keep an eye on... your KH is pretty low (I assume you mean 20 ppm KH and not 20 degrees). Fishless cycling can burn thru KH pretty quickly, ultimately causing pH to drop and slowing/halting the bacteria that oxidize ammonia. As long as you don't have fish in the tank, you can just plop some baking soda in the tank (a tablespoon or two would be OK for starters) to shore up KH.

Keeping KH stable once you have fish may or may not be a problem, depending on bioload, what the tap KH is, etc.

Good luck,
Jim

[Lukara]

JSchmidt, thanks for replying. Yes you're right in assuming that I mean 20 ppm for my KH reading. How low does the PH have to be before it becomes detrimental to the nitrifying bacteria?

[Lukara]

Okay I just went to "the Skeptical Aquarist" site and found the following info:


The pH is also a vital factor in nitrification. Maximum rates of nitrification occur at pH values above 7.2, peaking at 8.3 (a common pH for marine tanks) then falling at higher values. What surprised me was the rate at which the effectiveness of nitrification dropped in acidic pH values: to less than 50% optimal efficiency at pH 7.0, to just under 30% at pH 6.5, and to just over 10% of maximal efficiency at pH 6.0. At these low pH values, nitrifying bacteria don't die, they just stop metabolizing and reproducing...

Very very interesting... no wonder my tank took forever to to get this far in the cycle. :emb:

[JSchmidt]

You've found a great site (Skeptical Aquarist) that contains a high density of great info.

You list your tap water's pH as 7.6; is that right out of the tap or after sitting out overnight? The latter is the preferred measure...

Jim

[Lukara]

I agree that site is very informative. I don't know why I missed the information regarding the PH on this site when I visited it a while back :argh

Yes the PH reading of 7.6 is after sitting overnight in a bucket. My KH reading of 20 ppm was in the aquarium a week after my last water change. It is actually 30 ppm out of the tap after sitting out overnight.