Ok. I used to have a 10 gallon tank for quite a while and everything was fine. I have since upgraded to a 55 gallon tank and now I can't seem to keep my fish alive. I let the tank cycle completely before adding any fish and once I did add fish, for the first week everything was fine. So, after 1 week of healthy fish, I decided to add a few more and they were fine. Then about a week later I added a few more. Am I adding fish too soon? I was told I only had to wait 3 or 4 days between adding fish, but I opted to wait a week in between adding fish.
I do water tests weekly for nitrate, nitrite, hardness, alkalinity, and PH. All of my levels are dead on in the safe zone except for nitrate which always seems to border between safe and unsafe. (I have always struggled with nitrate even when I had my 10 gallon tank) But from what I have been told, higher levels of nitrate are relatively harmless and can lead to excess algae growth (which I don't have), so I'm thinking that is not the problem.
I have been told many conflicting things about my new 55 gallon tank. One pet store told me to do weekly water changes, but another told me that was too often and to do a water change once every two weeks. Which one should I do? Also, as with my 10 gallon tank, I plan on doing a monthly gravel bed cleaning and replacing the filters - is this ok for my 55 gallon tank?
I have some mollies in my tank and the pet store told me that I needed to add salt. So, I was told to add 1 tablespoon per 5 gallons. Of course when I do my water changes, I add less salt (ex.) if I do a 10 gallon water change, then I only add 2 tablespoons of salt to the new water). Am I adding too much salt or too little and could this be harming the fish?
My fish do not appear to be stressed. They are active and eating well. They are all peaceful community fish and I have never seen any fish nipping or picking on other ones. They all look great. They can be swimming around and looking all lively one day, then the next day I'll wake up to find one dead.
So far, I have lost 1
Guppy, 1 red wag
Platy, and just this morning, my only gourami. (It seems like I am losing one fish every other day now). And, my dalmation molly isn't looking so swell today. Could another fish in my tank be doing this? I have a mix of mollies, neon tetras, glass fish,
Platy's, and 2 dwarf frogs (NOT the clawfoot bulgy eyed ones, but the ones that are supposed to be safe and stay small).
Please help!

If you have any information that you think might help me save the rest of my fish - then I am all ears. I don't want to lose anymore. Thank you :)