Whoa! OK. Tanks (no matter what size or how many fish) SHOULD ALWAYS have a 20% water change ONCE A WEEK. NO less. NO MATTER WHAT WATER CONDITIONERS YOU USE!! Just because you use a water conditioner... it doesn't keep the water clean. A water conditioner only takes out harmful chlorines in the water. I use stress coat on EVERY water change to take out the chlorines and other things. I use stress zyme to help with my biological filter.
Now on the filter. A lot of people say to change it once a month or two months. A lot of other people say NOT to change it, to just rinse it off with bottled water. Personally, I change mine once a month.
pH in fish tanks is NOT as big a deal as most people will make it out to be. Fish will adjust to whatever the pH of the tank is, as long as its done slowly and they aren't just thrown in there. Then its a shock to the system. You SHOULD check these things AT VERY LEAST once a month. NitItes, nitrAtes and ammonia. I personally thing that checking for these once a WEEK is appropriate because you never know what is going to stress out your fish. Ammonia can pop up in a well established tank pretty fast.
Do you have a test kit raiquee? If you do what are your levels? What kind of problems are you having with your tank? You said "Not all fish die because they all are sick. Something is wrong with the water. And if my levels tested with flying colors when i went to get the fish, and i left it alone, and all my fish die, obviously something is going wrong. And this has happened more than once."
How old is the tank? Maybe your biological filter isn't kicking in. I have had that problem. You can set up your tank and have everything look perfect... then get fish and put them in, but your tank hasn't cycled yet, and your levels will fly off in all directions. I posted about cycling a tank in this forum if you want to read that. It might help. Let me know what is going on.