Your Local fish store should do a nitrate test for free. Without writing a whole article, I'll give you the ultra short version of tank maintainance. Fish produce ammonia (highly toxic). The bacteria that cover every SURFACE area of your tank (especially the filter sponge) convert the ammonia to nitrite (also toxic). Another species of bacteria convert the nitrite to nitrate (not toxic in low concentrations. The reason (one of them) for doing water changes is to remove nitrate laden water and replace it with nitrate free water, thereby keeping nitrate concentration low. If a tank is getting topped off repeatedly, the nitrate is staying in the tank and building up. If you fish died of nitrate poisoning I believe it will show reddness around the gill area (should be visible post mortum)
The bacteria mentioned above LOVE highly oxyginated water (aka your filter). As a consequence your filter media has a much higher concentration of these bacteria. When you rplace your media, or rinse it out in tap water (contains chlorine which kills bacteria) you get rid of those bacteria. Now there are not enough bacteria to deal with ammonia production, and you get an ammonia spike (kills fish and can cause cloudy water).
Not sure which if any of these issues is responsible without a more detailed account of maintainance practices and a water test. Let us know what you find. Good luck, and sorry for your loose.
