testing for nitrates is important if you plan on putting any invertibrates in your system. Inverts are much more sensative to nitrates than fish are. If you plan on putting inverts in your tank it is important that the bacteria for removing nitrates has had a chance to establish itself as well so your cycle, in that case, is NOT complete until your nitrates are also reduced.
I will agree with stemonia that about a 20% water change is all you should have to do at the end of the cycle (and for your regular water changes as well)
I disagree, IMO
, on only testing for nitrates, sg
. You should continue to test for amonia and nitrite as well as Ph. If something happens to your biological filtration, these will spike, and cause harm, long before you notice anything "odd" in a nitrate test. Especialy if your system normaly has low to 0 nitrate reedings. Nitrates are, as stemonia stated, a byproduct of bacteria turning ammonia to nitrite, then nitrite to nitrate. so if the process breaks down at the ammonia or nitrite level, your nitrate levels will not change but there is certainly a dangerous problem if the system break down at that level.