What causes NITRATES to be high?

Too much feeding, too many fish, not enough water changes.

How high are they?

Overfeeding, overstocking, dirty substrate, not enough water changes.

They said it. Ammonia from fish waste (and decaying matter like leftover food) gets turned into slightly safer nitrite by bacteria. Other kinds of bacteria turn that nitrite into even more safe nitrate. However, nitrate is still harmful. The exact effects are debated, but basically, your fish will be less healthy, shorter lived, and less likely to be fertile.

Anyway, nitrates can only be removed by water changes to plants using them for food. If you have a really heavily planted tank and/or plants that grow quickly (which uses up a lot of food in the form of nitrate, etc.), you can easily control nitrate buildup and even lower it to the point where you have to add it for the health of the plants.

HOWEVER, there are other pollutants that either are not easily or not able to be measured that build up when you don't do water changes, such as fish hormones which can cause serious health defects and stunting.

you might want to check you tap. i know in iowa around this time of year when snow starts to melt get lot of run off and my tap is reading 10-20 ppm in nitrates :jaw: but other wise what everyone else has said

Most of the bacteria needed to take nitrite and make nitrate reside in the filter, so if anything it would slow down the process to have the filter unplugged.

What's the tank's size and stock?

if you have dead fish or plants or the like it will also be bad news

Sounds like you could have some decaying bodies in there.