F8s keep dying?

[FallingShadow]

This is my first post so I apologize up front for any lack of info.

I run a 55g brackish tank, I will admit that I do not know the specific salinity or gravity however I go with the 1 teaspoon per gallon rule as I was advised by my LFS.

Nitrates=0 Nitrites=0 Ammonia=0
However about 4 weeks ago I saw a figure at puffer at my LFS who was just as fascinated with me as I was with him so I had to have him, no problem since they assured me my water perimeters where great. First 2 weeks he did great, large appetite, very energetic and responsive, all normal attitude, then within 3 days he started laying around a lot more then usual then one day I woke up and he was dead? I took him back to LFS and they checked my water and said it was all good? They guarantee there fish for 30 days after purchase with good water quality and so I got another and sure enough same fate. Idk what I'm doing wrong I figured 55 gallons and proper salting would be perfect for f8s?

Tank mates include:
3 Colombian sharks approx. 2.5-3"
Red tail shark 3"
2 veil angles 3" height 2-2.5" length
1 electric blue cray fish approx. 2-2.5"

 

[Brookster123]

You have all those fish in brackish? What kind of salt are you using? You really need to be checking the gravity.

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[FallingShadow]

You have all those fish in brackish? What kind of salt are you using? You really need to be checking the gravity.

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I'm using premium marine salt as I was advised to by people on these forums. I was told they would all thrive in a brackish setup and so far no one else has presented me with any signs of stress or illness other then my f8s and a rope fish I had previously, however I believe the ropefish caught an infection from my girlfriends GSP nipping him. I am currently trying to find a local store to provide a low end spectrum capable hydrometer. The petco and petsmart near me only have full salt spectrum and my local reef store doesn't sell any surprisingly.

 

[Brookster123]

So what's all in the tank?? What are your using for filtration? How often you wc? How long has this tank been running?

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[FallingShadow]

Everything listed above is what is in the tank, the gsp was from months ago before the f8s, when we moved the gsp to a new tank of his own we added the f8 to replace the ropefish and gsp. Filtration is 2 fluval 75 HOBs, I've always preferred 2x the recommended filtration as if one broke for any reason I have the back up, water change is 20% once a week very fine sand substrate so everything is easily spotted and removed, kept very clean. Fluval sky view 48" light, don't know if this is needed info but light as well include everything, is very much like to raise a F8 through its entire life but can bring myself to accidentally kill another one.