hello devs... thanks for responding so quickly... i REALLY appreciate it!!!

well, my tank has been running (in one form or another) for at least 3 years now. it is a 60 gallon. i have 1 small oranda, 3 medium orandas, 1 large oranda and 1 large lionhead... i also have a large apple snail. ok, if i remember correctly here are the parameters (i discarded the tests and they were my last ones... (oops!)

i know i am a tad overstocked but i do at least two 25% water changes weekly to maintain good water conditions... and i plan to get a new bigger tank within the next year or so...
ammonia- between 0 and 0.25 (SO hard to read on the test)
nitrate- 20
nitrite- 0.5
gh-300
kh-300
ph- between 7.8-8.0 (can't remember exactly)
my tank has been running these conditions for quite a while now... anyway, yes, i floated the bag with the pleco in it for around 20 minutes... then cut the bag and let some tank water come in and mix with his current water for several minutes before i released him into the tank...
(i had a hong kong pleco die the second (practically) he hit the water once... the explanation the store gave me was the ph difference... (understandable!) thats when i learned to slowly acclimate them to the tank water.)
the last time i saw him alive was i think tuesday or wednesday... he was only around 2-3 inches and he blended in quite well with the rocks, plants, etc...