Nitrate levels high and other levels fluctuating

[kmd0821]

The nitrate level in my tank is up close to the 200 mark. My nitrites are around 0 to .5 zone. My hardness fluctuates between soft and hard between 75 and 300. Alkalinity fluctuates between average 80 to 300 and the ph is going between 6.8 and 8.4.
What is causing all the fluctuation? How do I stop it and maintain a safe stabalized tank? What causes a high nitrate level?

I have a 55 gallon tank up and running for about 3 months. (never had these problems with a 20 gallon that had been running for several years and take much better care of the water in this tank)
This recently started fluctuating a few weeks ago, All the fish are fine but a gourami.
I have live plants.
Heated to around 76 degrees F
plenty of filtration (2 filters and an aeriator)
small piece of wood from a previous tank
only 1/2 in of gravel but many big rocks (should i use more gravel?)
I do a 20-25% water change (dechlorinated water of course) once a month and also add good bacteria between changes.
I feed once a day.
I have
3 baby platty's
1 pleco
5 zebra danio's
2 tiger barbs (just removed w gourami)
1 gourami (Removed to sick tank)
1 bala shark
2 clown loaches
2 cardinal tetras
2 rainbow things (small fish 1 1/2 inches max don't know their name)
1 black banded leporinus(bumblebee fish) (about 3 inches will grow to 8in)

 

[Dragonslair]

There are a number of reasons for your type of problem. Have you checked for dead or decaying matter including fish? Have you changed water company, or does your water come from another source? Has there been water works between your suppliers and you?
My advice toget a stabilised tank is maybe to invest in some Nitra-Zorb and maybe use RO water until you can determine the source of your supply. The condition you describe will give rise to an increased bloom in algae if left unattented and you may find your phosphates levels may have risen too.

 

[kmd0821]

the water company has changed, never gave it too much thought. Scary to think what may be in the water!
thank you for the advice I'll give it a try