More info please? What do you have in the tank? How big is it? Do you have driftwood? What are the parameters? What are the parameters of your tap? It could be a lot of thing but until we know more info we're not gunna be able to help. A pic of the tank would help.
Sorry, I have another thread going here too but I thought this was more specific.
There's pics in this thread.
http://www.aquariumadvice.com/forums/f12/thought-on-my-55-gallon-suggestions-pics-134009.html
I have
FISH
1 Pictus Catfish
5 Red Eyed Tetra
6 Zebra Danios
1 Blue "Three Spot" Gourami
1 Gold "Three Spot" Gourami
PLANTS
2 Java Fern
2 Anacharis
1 Banana Plant
1 Wisteria
I tested the tap water straight from the tap, and it was like 8.4
I did take additional tank samples from deeper down, and it was a little higher, but not much.
I thought maybe the heat from the lights at the top affected it.
You should also check for ammonia/ nitrites. If you have not seeded the tank, it might just be finishing cycling at 5-6 weeks. Ammonia is basic, so the pH is increased during the ammonia phase of the cycle. Once that is done, the pH can drop back to the "normal" value. <I am assuming that your tap water has low buffers & is naturally acidic, but had been limed to bring the pH up (water co does that to prevent pipe corrosion) ... the effect of liming does not last long & the water will revert back to "original" ... hence the suggestion to test the pH of tap after sitting out for a day or 2.>
I've been checking all the essentials on a daily basis, but in the last 2 days they have moved a lot as well...
I thought I was nearing the end of my Cycle, but maybe not.
I was almost near Zero-.25 as far as Ammonia and Nitrates go.
I've been at 0 PPM Nitrites for the whole 5-6 week life of the tank. Though on 4-5 random days the Nitrites have been .25PPM
Now the Ammonia is around a 1PPM, however that's what the tap water Ammonia shows right out of tap.
So I thought I've been knocking down the Ammonia with fresh tap water, but it's the same level (1PPM)