Sick Cory(s)

Soo im very new to this, but i think i have a basic enough understanding so far. Here's the story: I went on vacation and under the care of my neighbour one of my spotted corydoras passed away. When i found it it was mostly ingested by my bloodfin tetras, so im not sure if they killed it or something else did (im guessing something else, although my bloodfins are QUITE vicious). a few weeks later one of my corys girls started to lose colour and he stopped eating. i tested the water and found high nitrites and ammonia so i took care of the problem. then out of no where one of my corys with no discolouations and no obvious behavioral issues passed away as well. the water is now of great quality, and my previously sick cory is active and eating, but still has a shiny greyish gill colour, although it is getting better. i really hope little "rajon" can pull through, as shabaz and dejuan did not =(. any suggestions at this point? im assuming he needs friends as they are a shoaling fish, and im going to keep track of the water religiously. i have been doing water changes each weekend. thanks for the help -pkppv

15 gallon tank:
6 silvertip tetras
4 bloodfin tetras
1 (of initially 3) spotted corydora

all 3 corys were VERY happy until the past few weeks, they were incredibly active, ate very regularly, always enjoyed each others company, etc.

possibly overcrowded as well... I've only been in the hobby since January so any help would be EXCELLENT! thank you