Rising nitrite

[yoink]

Hey all, I'm new here to the forums. Just looking for some suggestions on what to do about my tank.

It is a 29-gallon with 9 tiger barbs and a rainbow shark. Ph 7.5, Gh 6-7, Temp is 78. Ammonia is always at 0 ppm nitrites have always been at 0 ppm since cycling about six months ago. Nitrates are usually 20-30 ppm. A few of the tiger barbs have developed a slight case of what I believe to be fin/tail rot.

On Saturday I preformed a 25% water change and added Maracyn-2 and Maracide as recommended at the local fish shop. I followed the direction on both products, It also stated that they were ok to use together since they are both made by Mardel. On Sunday I added another dose of Maracyn-2 per the directions, the Maracide is only added on day 1,3, and 5. So today I go to give them some medicine and I notice that the pump/filter is not running. I unplugged it and cleaned it out (did not touch the bio media) and plugged it in. It fired right up, so I decide to question the wife as to when this could have happened. She fed the fish this morning at about 8:30 am; the pump was working fine at this time. I notice the pump not running around 2 pm. I test the water to make sure that everything is ok in there, and the nitrite level is around 2 ppm.

Could the nitrites have possibly risen that much in only a few hours of not having the pump/filter running?
Could the medicine have killed off the beneficial bacteria, anyone use this stuff before?
Could the medicine cause my nitrite reading to report a false positive?
These are the only things that I could imagine happened. These fish have been in the same setup with no fish added or subtracted for over six months. I use an Aquarium Pharmaceuticals test kit.
Any help or suggestions are much appreciated. I will be doing a large water change and adding a little aquarium salt later tonight just in case it is actually that high. Thanks.
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[Damon]

Maracide doesn't kill of bacteria. Thats a LOT of barbs in a 29 gallon tank. WHat type of filtration do you have? I would guess going by the limited info I have on your tank that the tanks biological filtration is inadequate.

[yoink]

I have a marineland filter with a biowheel, it is the one that says it will filter upto a 50 gallon tank. These fish have been in this tank for over six months with no sign of ammonia or nitrite, tested weekly.

[Damon]

A sudden rise in nitrites would indicate an increase in waste or a bacterial die-off. Have you cleaned your filter recently? Is the bio-wheel still spinning?

[yoink]

The biowheel is always spinning, although it was not spinning when the pump stoped working earlier,but it never completely dried out. That was my initial fear, that all the bacteria has died off, but I tested again tonight and still no change in perameters. I am going to do a 50% water change and hope it will not continue to rise.