tank going through another cycle?????

[mdk2424]

Ok I had the tank set up since october went through the cycle in late december.
water conditions
amonnia 0
nitrites 0
nitrates 10 FOR SOME REASON NEVER GOT ANY HIGHER
ph 7
doing 5% water changes a week in a 29 gal tropical fish tank stocked with 18 fish all was good fish were happy and now there dying.
2 weeks ago ph droped to 6 and the next day amonnia .50 then every thing went down hill since.
amonnia 8
nitrites 0
nitrates 0
ph stll crashing 6 / 6.4
been doing 25/40% water changes a day to cut the amonnia down but nothing seems to be working. SEEMS LIKE I AM GOING THROUGH ANOTHER CYCLE BUT WHY????????????????????????????
Please and help or advise to save my sons fish would be helpful thank you.

[MySpace Mike]

Size of tank?

You may have overstocked it or stocked it too quickly and the overloaded the bioload that it could handle.

[Rbishop]

Are you using a good liquid test kit to get those readings?

What are the same readings on your tap water? Your tap water pH test should be on a sample that has set out overnight in a shallow dish.

Are you using a water conditioner, like Prime, to lock up that ammonia?

You mentioned 18 fish in a 29 gallon tank..what kind of fish?

[KarlTh]

Low pH inhibits the filter bacteria. I'd advise getting it back up, but slowly because at the moment the low pH is preventing the ammonia from poisoning the fish. Change it quickly and they'll croak within the day.

First things - what is the pH and KH of tank and tap?

[7itanium]

somehow it has went into a mini-cycle

the only thing I can think of is overstocking and low PH