low ph level

Question
I have a 30 gallon tank.  it has been in operation for about 2 months now and i have been gradually adding fish without much of a problem (just an ick outbreak initially, but nothing since).  my tank now has 1 small pleco (the local fish store assured me it would remain small, around 1-2 inches) 3 neon tetras, 3 red tetras with a small black spot, 3 silver tetras with a similar black spot, 3 small bloodfin tetras, and a recently added pearl guourami as well as a ram.... the ph is hovering around 6 and the tank recently had very normal levels for everything else.  i have one hanging filter where i change the charcol once a month and rinse the foam filter as well (but not at the same time)  i change 20% of the water while vacuuming once a week.  i just wanted some re-assurance on several points.  first, is my tank overcrowded?  i went through a lot of trouble to gradually add fish and it seems that even at the current level i should maybe be able to add another 3 neons to get to a full tank.  also, what is the best way to normalize the PH - it was reading normal until just about 4 days ago, just after i added the new fish.  any advice would help, as well as how often to wash/change the foam insert - it gets fairly dirty but i know that rinsing it too often can destroy the biological filter.  thanks,
steve

Answer
Hi Steve;

The population looks good actually. Wait at least a week for the system to compensate for the last new fish and you could probably add another 3 neons. You may have to return the plecostomus though, depending on what kind he actually is. There is no such thing as a plecostomus that stays a couple of inches long. They all get to be four inches and larger. The common pleco, the easiest to find and buy, gets over a foot long. Yikes! The good news is, they may have given him the wrong name. What you may have is a little fish called an "Otocinclus" also known as "The Dwarf Suckermouth Catfish". It's the only algae eater that stays under 2 inches long. You will probably need two more though because each Oto can handle about a ten gallon tank each. Here is a profile page about them with pictures;

http://www.fishprofiles.com/files/profiles/615.htm

Your maintenance is just fine, including the filter. The pH should be left alone though. It's dangerous to try to change it with chemicals or additives because it causes fluctuations. What your tank may need is more oxygen. I've had tanks with chronically low pH that goes up again after I add bubbles to the water. An air pump with an air stone or an extra filter that agitates the to top of the water more would be helpful. Also, test your water at different times of the day. It can fluctuate mildly over several hours between morning and night. It's natural and very gradual so it doesn't affect the fish like a chemical that causes quicker changes.

At Your Service;
Chris Robbins