What diseases do you always seem to run into?

I would say that the number one disease that I tend to see in other fish tanks (the stores, friends, or even those in groups) is Ich. Without a doubt it seems to be the most common, followed up by popeye for some odd reason.

The dreaded ich.

Ich from some at pet shops or they have fin rot or as super bloated for whatever reason.

The very first time my fresh water fish suffered from Velvet, it was a very horrible experience for me because it almost wiped out my fishes. I would never like to experience that again.