New fish dying

Question
Hi, we have a 12-gallon Eclipse freshwater tank that is 1.5 years old.  Every time we add new fish, they are dead within a couple of weeks.  Most recently I added four that died within three hours of being put in the tank.  They were hanging out at the surface and then started to turn upside down and lose the ability to swim.  I have tested the water (Ph, nitrates, nitrites, ammonia, etc.) and everything appears normal.  We keep the temp at 75, we change 25% of the water and replace the filter once a month, we are careful not to overfeed.  We have one neon tetra left from our original batch of fish, but everything new that we add dies.  We did not have this problem when we first set up the tank. I have been told that the new fish are stressed, but it seems unlikely that every single one would die.  I have also been told not to use reverse osmosis filtered water, after being told by someoen else that that type of water was the best to use.  We are careful to acclimate the new fish to the new tank.  Any ideas?  My children are young and it is very depressing to have every new fish die.  

Answer
HiYa!

Ok first things first, if you have a 12 gallon tank, its only a smallish tank. Small tanks are very easy to go off track with, larger tanks are much easier to maintain.

You say you change 25% of your water once a month and replace the filter once a month? To start off with you should be doing a 25% water change every 10 - 12 days not once a month, it just simply isn't enough. Fish benefit greatly from frequent water change,you will find that if you start doing 25% every 10 - 12 days the health of your fish will improve immensely.

When you say your replacing your filter, I presume your filter media, not your whole filter? If you are replacing your filter media once a month this could be where another problem lies. When tanks are first cycled, they tank from between 2 - 6 weeks to fully cycle, this means it tanks this amount of time for the ammonia cycle to occur and beneficial bacteria to grow in your tank, which every tank needs. You are replacing your filter media just at that time when you beneficial bacteria is probably getting to a healthy level and replacing it with brand new stuff that has no bacteria on it whatsoever, so effectively every month your starting your tank on a new cycle, so your fish aren't going to survive.

You should be 'washing out your filter media' every second time you clean your tank. So say you water change next week, then the next time you water change after that, take a bucket of water out first, take your filter media out of the filter and rinse it in the water from the aquarium in the bucket, that way you are cleaning it but not getting rid of any useful bacteria. Your filter media dependant on your filter, should be changed every 3 - 6 months, some of our filters we have had for over a year and haven't changed the filter media once and haven't had a problem!

Also when you water change, you NEED to gravel vac, this means using an actual product called a gravel vac that cleans the gravel as you move it along the substrate and takes out water at the same time, not everyone does this when they water change and I can't express enough how important it is to keep your substrate clean as well as your water!

Hope this helps!

Cheers! Rach