I need HELP with my 75 Gallon Tank!!! ASAP!!

I am having some REALLY bad luck with my 75 Gallon fish tank, and I don't know what is going on. Heres a little info about my tank....It has a Fluval 350 & a Magnum 350 Canister Filters, UV Steralizer, Heater (Set at 80 Degrees). The Fish that are actually LIVING in the tank right now are 2 Figure Eight Fresh Water Puffers, 2 German Blue Rams, 2 Red Tailed Sharks, 2 Siamese Alage Eaters (Who DON'T Eat alage LOL), 2 Petracola Catfish and some Ghost Shrimp. The tank is about 4 months old, we had did the fishless cycle with amonia for a month until the parameters were literally PERFECT!! We wanted to make sure we did this right, because we wanted to get some of the more expensive fish and didn't want to have any problems (Which of course we are). Also the tank is planted, has drift wood, lava rock, gravel and some river rock. Everything was boiled. The parameters are still fine, and we do a water change on the tank 1 a week. Due to the plants and the fish and how messy they are. Daily we feed the fish Hikkari (SP?) Frozen Blood worms and Flake Food. A couple of times a week we will also give them Frozen Brine Shrimp (None of our LFS carry live...Weird I know), and Frozen Krill as a treat. We make sure to only give them what they will eat...which they eat ALL of it!!
Here is my problem...the fish that were in there had all been in there for 2 Months...NOTHING new was added. No Plants, fish, nothing. Last Thursday my Boyfriend noticed our Blue Rams Eyes were Popping out, so we transferred him to the 10 Gallon Hospital tank where he is being treated (Still to this day). Then that SAME night out Blue Paradise Gourami was sitting at the top of the tank not swimming. He was to the point where he was swimming on his side. Please note that prior to this, about 3 weeks before this he had somehow hurt his lip. It looked like he literally ripped it off. So we added Salt to the aquarium and his lip had started to heal, and he was eating and his normal self. Back to the point, we tried feeding him and he wanted nothing to do with it. So we took him out and put him in the quarntine tank as well. We noticed that his lip looked like it was black, were he had hurt it. The next day I cam home from work and he was nose first in the gravel dead. I have NO idea what happened to him. Then Thursday night we noticed our Gold Gourami was pooping white (Sorry to be so gross), Our gourami usually poops Brown and BIG LOL he eats EVERYTHING in site. So we tried to feed him, and he would have NOTHING to do with it. This was NOT normal behavior for him and we knew it. My boyfriend had tried for an HOUR to catch this fish on Friday morning but couldn't get him and had to go to work. When I got home from work, the white stuff was ALL over the tank. I immediately did a water change and sucked it ALL OUT, I made sure to leave nothing behind. That Night my Boyfriend turned the stearlizer on, and we spent 2 Hours trying to out smart this fish. We finally got him and put him in the hospital tank. The white slowly started to turn to a clearish white (Which thank goodness he swam past the filter most times and it got sucked in there lol). I could NOT get him to eat, so I put some brine shrimp in there to make him eat. Which he actually did eat some of that. His belly was bloated but had been since the day we got him, he is always eating ALL day and has always had a bloated stomach (and we didn't think anything of it, we thought he was just a big fish). I thought he was actually going to make it, and was getting better. He NEVER lost his color and was always swimming around. Finally last night he went to fishy heaven. I am pretty sure he had a parasite. This is our 6th fish from this tank alone to die. 1 other Gourami died of stress, another died of a parasite, 2 German Blue Rams died from Ich under there gills they got from the LFS, and the other German Blue Ram I have no idea what he died from. He lost his color and his aggressiveness and was dead 3 days later with no other signs of sickness. I have been watching all of the other fish now in the main tank and everyone seems fine. We also bought some Immune-Plus that we are now adding to the tank. Does anyone know what is going on with my tank???? Or maybe what I am doing wrong here?!?! I need Help!!!!

The Figure Eight Puffers are actually Fresh Water puffers, the Green Spotted are the ones who slowly go from fresh to brackish as they age. We have done much research on this and we have also asked our LFS who have said many people DO think they are Brackish which they can be. But if you read online Many people have expirenced that Figure Eights due better as Fresh water. Are Puffers have been in fresh water for 3 Months now and are VERY healthy and actually are doing better than most of the fish in the tank. Anyways are puffers aren't aggressive at ALL, they are the least aggressive out of ALL of the fish in the tank (Yes I know this suprises people but we also have dwarf puffers in a seperate tank and they are meaner than these ones). Figure Eights are known as the more friendly less aggressive puffers. The rams Chase the puffers and even RAM them, the puffer just swims away or keeps hunting for food. The Sharks argue with themselfs and chase the alage eaters. And the Rams mainly chase eachother. We do about a 50% Water change once a week, we use AquaSafe by Tetra Aqua with BioExtract for our water conditioner (Which we use on all 6 of our tanks. We also only feed the fish once per day, and we feed them a small broken piece of the bloodworms which all gets eaten in less then 5 min. and a pinch of flake food. I literally just tested our water 2 miniutes ago with The API Master Freshwater Aquarium Testers (Not the Dip Sticks, I don't trust those). Here are my Parameters.... Ammonia-0ppm, Nitrite-0ppm, Nitrate-0ppm and PH is at 8.0 (are PH in all of our tanks is always high).