at a lost

Question
I have had a fluval edge set up for 8 months. It has only one dwarf orange crayfish and one nitrite snail. Every thing was fine,then my crayfish died for no reason.I test my water twice a week with api liquid test kits. Ammonia is always 0,nitrite is 0,nitrate is 5.0,copper is 0.Oxygen is fine and water is very clear with no oder. My filter turns over my tank 19 times every hour. I do a 20% -25% water change every week. Temp is also good. So then i put in three neon tetra and after a week one of them died,the other two are very active and doing great.I know my water perimeters are fine and i never over feed. What do you think.Thank you so much for your time.

Answer
Don,

Unfortunately, sometimes we don't know why our fish die. It sounds like everything in the tank is very good so maybe the crayfish was old or just had something that didn't show. As far as the neons, sometimes you get them sick. Since they are really not taken care of well at the breeders or the pet stores, it is important to quarantine the new fish for 2 weeks in a hospital tank to make sure they are ok. If they look good after 2 weeks then they can be put in the community tank. This helps ward off any problems that might arise with the fish already in the tank. Sorry I can't be of more help but sometimes we just don't know why they die.