6 fish died today, huge spike in ammonia and nitrites

[steelo]

I've read that hydrogen peroxide can crash a cycle but have no experience. What strength was the solution?
Once tanks are well established (1+year), using tap water on the filter media doesn't disrupt the cycle so there will be something else that caused the ammonia.
It's promising that your test results are staying safe so keep up what you are doing.

Why are you giving up on neons if they are your favourite? If you revisit @Colin_T s questions he may say that they are viable? Knowing the pH, GH and tank dimensions will help for future fish suggestions.

It was 3% off the shelf Wal-Mart 'Equate' brand. I did VERY thoroughly rinse everything cleaned with hydrogen peroxide with water but who knows...this very well could have been the culprit.

I just haven't had good luck with neons. I'll get 6 or so and they will be doing great, then months later appear to slowly waste away. This is a 45 gallon tank that has been running for 5 years, so they should have plenty of room and the water fairly stable (up to now).

I don't pay much attention to the Ph because the other fish never seem to have an issue and I use the same source with every change (city tap water and dechlorinator added to the tank) I seem to remember doing a test years ago and finding it was pretty hard water.

 

[seangee]

I performed my usual 25% water change last Saturday but really focused on removing gravel waste.

pH has not been mentioned anywhere in this thread. If you disturb the gravel and it has been undisturbed for a while the toxins that have been accumulating in the gravel are released into the water and are likely to cause a pH crash. This is far more likely than an established tank suddenly "losing" its cycle. It has happened to a few members on this site. If you don't clean your gravel thoroughly every week and decide to do a major clean up its best to remove the fish and follow the gravel clean with an 80-90% water change, preferably with a second large change the next day.

 

[steelo]

pH has not been mentioned anywhere in this thread. If you disturb the gravel and it has been undisturbed for a while the toxins that have been accumulating in the gravel are released into the water and are likely to cause a pH crash. This is far more likely than an established tank suddenly "losing" its cycle. It has happened to a few members on this site. If you don't clean your gravel thoroughly every week and decide to do a major clean up its best to remove the fish and follow the gravel clean with an 80-90% water change, preferably with a second large change the next day.

I normally do a quick cleaning every week of the gravel where I siphon waste sitting on top but I occasionally get lazy. I try not to dig deep in the substrate which, as you say would release toxins. Where I had a 'crash', I was much more thorough and siphoned deep. I will from now on make sure to do at least give a quick gravel siphon once a week.