My populated tank has been running nicely for 4 months or so. I do a weekly test with the API freshwater test kit and a weekly water change of 25-30% (63l tank). My normal readings have been 0.25ppm ammonia, 0 nitrite and nitrates in the range of 5-20ppm.
Last week there was a sudden nitrite spike up to 5ppm. This coincided with some unusually inactive fish and others (especially harlequins) spending a lot of time at the surface. I did my usual water change and also added a higher than normal dose of Prime to try and prevent nitrite poisoning. They all survived and I did an additional midweek water change and test and nitrite was back to 0, but also nitrates were zero. I also cut back on flake food quantity to try and avoid waste food turning to anything harmful.
When I did the regular test and change this weekend, ammonia was up to 4ppm, nitrites still zero and nitrates were zero.
Any ideas on what is going on? Especially with a new surge in ammonia and an unusual lack of nitrates? I wondered if I had adversely affected the chemical cycle with the emergency prime dose.
Views and advice welcome. Is there anything else I should be doing to restablish my stable tank?
Many thanks