I think I am in over my head!(Sorry for the Rant)

I think I got over my head a little bit in the aquarium trade. For the past 2 years I kept a ten gallon, and managed to keep a small school of black tetras alive. We just moved into a home that we are going to be in for a while, so I decided to delve a little deeper.

I started off with a 40 gallon that came with two black tetra's and a pair of Bristlenose Pleco's, that ended up being a breeding pair. My wife decided we needed some guppies and mollies to go along with these fish, and we were already at the filtration limit. Once there, our Pleco's gave birth, and 8 fry survived. This brought me to tank number 2.

Tank 2 was a 48 gallon that I moved my pleco fry, as well as my guppies. Since i now had a little breathing room in my main tank I got a pair of angels, and I am upgrading the filter so I can sneak in one more angel.

I also found a trio of badly neglected Pirahnna's with a 50 gallon tank that is also now blessing our home.

Onto the issues...I just had a baby, and got a little behind on my water changes. During the past 2 weeks, my guppy tank all went belly up, but the pleco's were fine. I checked my water levels, and my Nitrates were a little high, but ammonia and nitrite's were zero, I also didn't notice any parasites, and they are cheap fish, but I still feel bad losing an entire tank. During this time, I also forgot to double check the bucket after water changes to make sure none of the fish were stuck to any decorations. Yesterday I went to do my weekly changes, and found my female pleco dried out and very dead. I also had two mollies jump out of the tank(which I realize it was probably the angels).

I think I am back on track now, it is just very frustrating!

TL:DR: In the past two weeks I have lost an entire tank that had good water, two of my mollies, and I accidentally killed my Pleco. I