Rise In Harmful Chemicals?

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

The fact that you say you "normally" get 0.25 ppm ammonia is of some concern.  After 4 months, you should never get anything but zero! I suspect that Prime is your problem.  The Prime FAQ gives some info about the incorrect figures given for ammonia tests when used with Prime.  It is imperative that you perform your ammonia tests before adding prime or wait at least 24 hours after adding it.

I tested again,ammonia still 4, nitrites 0 and nitrates 5 (so encouraging that nitrates are now showing?)

I did a large water change as suggested, so will test again tomorrow.

I used a different dechlorinator to try and avoid a Prime false positive.

As for the cause, I can only think it might have been overfeeding - perhaps if the kids did it unsupervised when I was late home from work!

Thanks for the advice.