Aquarium snails dying or what,is my african cichilid killing my snails

[pufferlover]

first of all sorry my question is so long,but i feel you need to know all the facts to make a good assesment

im a beginning fish keeper,i originally bought two tanks for my kids one a 3 gallon starter tank and one a 5 gallon hex tank in the 3 gallon i had one orange colored african cichilid and one 3 spot gourami along with 3 snails one apple snail, one grey looking snail,and one small brown snail.the snails thrived for around 4 months and did quite well living in the tank with the fish.
now heres where it gets wierd to me. in my five gallon hex tank i had one marble angel fish, one tetra,a spotted puffer,and a black pleco who later died from a disease that ate his eyes believed to originated in the store i bought him at
anyhows i knew my puffer needed more room so i bought a 30 gallon tank,and cycled it.i at first tried to place all the fish and snails in the same 30 gallon tank but the gourami who was the largest fish of all was really bullying my spotted puffer to the point where he wanted to jump out of the tank and tried several times.so i removed him and the cichilid since they lived together before with no problem and put them into the 5 gallon hex tank.mean while i noticed my snails who were doing fine in the 3 gallon tank with the gourami and cichilid were now lying dormant at the bottom of the 30 gal tank all except the small brown snail who seemed to be doing fine until he disappeared all together,i still dont know where he went unless i accidently buried him in the rocks.figureing the apple snail and the greyish snail just didnt like the water in the 30 gallon tank i then put them into the 5 gallon tank back with the fish they originally were with.the grey snail never came back and i eventually died so discarded him.the apple snail did come back out and i seen him for about 3 days roaming the 5 gallon tank then suddenly he was back at the bottom lying dormant again.when i picked him up to check to see if he was alive pressing gently on his bottom hatch a white milky liquid seeped from him and he didnt flinch or move,i figured him dead and discarded him.
i knew i needed something to eat the algae from my tanks so i bought a dark greenish colored pleco and placed him in the 30 gallon tank with the angel and the puffer and hes been doing good since.
i still had nothing for the 5 gallon tank and i knew from previous experience that my african cichilid would not let a pleco enter the tank as he tried to kill my previous one once before,and may have even been the reason for his missing eye and not a disease.so i bought some algae controller liquid and put it in the 5 gallon tank,but it didnt seem to work as my rocks are starting to get covered in algae.so i bought a rather large brownish snail and put him in the tank,and for a few hours he roamed about it,however i did notice my cichilid picking at him and i believed him to pick off one of his tentacles and eat it,but i left him alone as he seemd unfazed by the picking and the cichilid left him alone for a while...i went to take a nap and when i awoke i checked only to discover him lying dormant at the bottom of the tank. i observed for a time and noticed the cichilid ramming at his shell with his snout,so i figured id better get him out of there that the fish was trying to eat or kill him.i picked him up, poked at him and confirmed he is still living,so i then put him into the 30 gallon tank in hopes he would return to life,but he hasnt as of yet.
my question is: will he return to life,and will a african cichilid eat snails
also the spotted puffer i have requires briney water,and i have been putting fresh water salt in both tanks can this hurt snails.like i said the others seemd fine in the 3 gallon tank living for 4 months in it and it also had also had salt in it...i dont understand whats killing them, the cichilid never seemd to be agressive to them before in the 3 gallon tank so why now in the 5 gallon..can anyone help i really need a algae eater to control the algae in the hex tank pls help

[pufferlover]

Additional Details
ok to get it straight i said i was a beginner but iam not dumb,and in the 6 months i have had the fish i have learned and read up alot on them. i have a test kit here are the results:
ph- 7.2
high range ph- 7.8
ammonia is a bit high at 0.25-0.50 and i just added an ammonia tab to correct this
nitrate- 10-20
nitrite- 0.25
i also added a correct ph tab

again the puffer requires brackish water and is in a 30 gallon tank which is the tank the snail is now residing in safe away from the cichlid

There are only two fish in the 5gallon tank

i perform 20 percent water changes regularly every week,and i clean the tanks rocks once every two months ..i also add a de-chlorinator to the water at every 20 percent water change