How long has your tank been set up? -if it's newish, then you are almost at the end of your cycle, if it's been set up for over a couple of months then perhaps you've got too many fish and your filter can't keep up with the waste your fish are producing.
High levels of nitrate are nt -toxic- to fish but they are an indication of poor water quality, (either not enough water changes, not enough detritus being taken out during cleaning or you are feeding your fish way too much). It's not so much the nitrates, but the other stuff that builds up in the water as well (other organic compounds, fish growth retarding hormones that are released by the fish, bad bacterias etc) that can damage your fish, I like to think that testing nitrate is a good overall indication of the levels of everything else that's going on in a tank, not just strictly the nitrate levels. (I know this is not strictly correct, but it's more of an indicator).
The closest the nitrates are to 0 the better the water quality you are keeping for your babies.
Hope this helps!