Biological Filter - Dormant or Dead?

[fry]

What is the likelihood of having a biological filter "die" or go dormant under the following tank conditions?

20g tank
ph slightly lower than 6
nitrite 0 ppm
6 dGH (around 107 ppm?)
temp 81
ammonia/ammonium reading is around 2.
I don't currently have a test kit for nitrate.

Fish are healthy - vibrant colours, lively and active.  (Chocolate gouramis, pearl gouramis, cardinal tetras, emperor tetras, lemon tetras, otocinclus).  No signs whatsoever of discomfort or ailment.  Plants are numerous and growth is fairly vigorous.  One piece of mopani, CO2 unit, ac 200 filter.

Water changes have increased in percentage since the ammonia showed up, but occur once per week.  (Yesterday's w/c was about 60%, usually 20-25%).

I ask this now because I lost a bushynose pleco last week (healthy looking at lights out, round little belly, dead in the morning), which devastated me - first fish death in that tank.  The associate at the LFS did not honour the 15 day guarantee because my water was "so terrible", and he informed me that I had "killed my biological filter".  (Personally, I would have thought the chocolate gouramis would have been the first to show stress if the water was as bad as all that.)  

I am attempting to raise the ph a little, and to get rid of the ammonia/ammonium through water changes, but I'm curious about the state of  biological filter?  Any help or insight would be appreciated.

I am not new to fishkeeping, but not a water chemistry scientist either.

[dpatte]

the ammonia should always read 0 or near 0 in a tank with an active nitrogen cycle.

Either you cycle is wacko or your test kit is.

[artw]

when you clean your filter media do you just replace it?
do you wash it under tap water?
do you aggressivley clean surfaces in the aquarium?
has your tank only been running for 15 days?
if so, then everything is normal. it can take up to 6 weeks for a tank to cycle the old way.
patience and it should clear up on its own.

[artw]

and thats another thing.
fish stores that offer 15 day guarantees on fish and then they sell the customer a new tank and 20 fish.
well of course the damn things are going to die in 15 days and of course the water is going to be crap!!!!! (hello Ammonia)!!

I don't get it.
*shakes head in discgust*.

[fry]

When I clean the filter media - about once a month - I just squeeze it out a few times in water I take from the tank during a w/c.  Just enough to get the major detritus out of it.  There's currently nothing else in the filter other than the one sponge/foam (and another new sponge I'm seeding for a new tank).

I rarely clean the surfaces of the aquarium - my otos take care of what few green algae spots there are.  

The tank has been set up this time since the first week of July.  I've had the tank for about 20 years, and except for the last 4 years I've had fish in it constantly.  (I had a 48g with large CA cichlids collapse on me due to a faulty stand 4 years ago, and I've only had the nerve to get back at it since this July! It was devastating, let me tell you, not to mention costly and very, very messy.)

After the 50% w/c on Tuesday I took the ammonia reading again yesterday evening, and it's dropped to 0.25 now (phew), but I'm still curious about the dormancy of the biological filter in low ph water.

Thanks for your reply Art.

[artw]

well it's quite possible the nitrobacter or somonas or whatever stop working in low ph environments.
glad to see you got your tank working out

you've been doing this for 20 years and the cycle concept hasn't changed since then eh

[fry]

Well, call it blissful ignorance, lack of proper testing equipment, or what have you, but I've never encountered the possibility of killing my nitrogen cycle before!  Go figure.   :roll:

If anyone knows of any good online articles on this issue, I'd appreciate getting the links.

[Sue]

http://ovas.ca/article.php?sid=83

above article on ovas home page looked pretty good