I'm just gonna say this, break down one of the 20 gallons if you're going to go with a 55 gallon. I have four tanks currently, a 5 gallon betta tank, a 10 gallon quarantine, a 15 gallon planted tank and a 55 gallon planted tank. At first I kept up on everything pretty nicely, never missed a water change, always checked the parameters, and after a few months it got tiring. It became more of a hassle than enjoyment. Don't get me wrong, I love my tanks, but I work two jobs and a needy girlfriend lol. It's often difficult to find time to push everything aside and work on each tank, meaning water changes, dosing ferts, cleaning the tank, removing algae, trimming, feeding the fish, checking parameters and so on. It takes a lot of work, that's why when my betta dies and I'm done getting new fish, I'm tearing down my 5, 10 and 15 gallon tanks to make room for a 20 gallon reef tank. In the end, I'll only have my 55 gallon and a reef tank to care for.
However, as mentioned above, take care of those tanks first, you have ammonia and nitrite reading, even though they're tiny readings, your cycle isn't stable. You may think it's because a fish died, but honestly Baylor, I'm almost certain it's your cycle. If there is ammonia and nitrite, then it means your nitrosomonas and nitrobacter colonies aren't able to keep up with the bio-load. You've been dealing with this problem for a while, and your fish have been suffering under the conditions.
If one day, you do decide to go with a 55 gallon tank, make sure the cycle is complete, after it's done cycling, just wait a month or two before adding any fish. Let the tank age a little bit. This is what I did when I set up my 55 gallon, and not once have I dealt with a dead or sick fish.
