Hi C&D,
You most probably have had an ammonia and nitrite spike, but assuming you have a mature cycled filter, you most likely won't have noticed as your filter will have dealt with it pretty quickly, leaving only the nitrate.
If your tap water has virtually no nitrate, just do a few partial water changes to lower the level. I don't think there is any need to do large water changes (50% or more), as the most recent scientific evidence suggests that most fish will tolerate nitrate levels of 400ppm without negative effect, so 100ppm isn't really too much of a problem, unless you are keeping very sensitive fish, like Discus.
Also, remember that large swings in water parameters can be detrimental to your fish. Normally, large water changes don't present this problem in a well maintained tank, as regular water changes keep the tank water parameters roughly the same as the tap water parameters, but in this case, your tank nitrate level is high and tap water nitrate level is low. Large water changes at this point could drastically change the water parameters and may do more harm than the actual nitrate level itself.
If it were ammonia or nitrite which were high, i would recommend that large water changes should be performed, as these toxins are far more deadly to fish than the change in nitrate level, however, as nitrate is relatively non-toxic, reducing it slowly will be more than sufficient.
I would suggest that you start with a 25% change every other day, to reduce the nitrate level slowly. Once nitrate levels in the tank come back in line with the water from the tap (or there-abouts), you can cut back to 25% weekly, or whatever your chosen water change routine may be.
Hope this helps you.
Cheers
BTT
), but knowing what your actual readings are would help!
? so i assume nitr I te is 0??
