Ok here is the situation. I have a 55g tank that has been set up for over a year. Originaly I have a had a Common Pleco for two years and recently he some how hurt himself in the tank and died. He had a scrap on the top of his head all the way to his upper fin. We purchased another Common Pleco that was about 3 inches long and also purchased some Green Tiger barbs at the same time. After about a week in half the Pleco died, Tiger barbs still alive. Went and purchase another Pleco this one larger about 5 - 6 inches long. After a week he had died. I have done a water check and see not issues with any levels. I have not put salt or added a salt to my tank in over a year, just when I first set it up over a year ago. I als drop in agea wafers for suplaments. I have two other tanks a 40g and 10g. The 40g has two Plecos in it and had no issue introducing eitehr one. Need some help on this? Only thing is the second Pleco did look kinda old??