Fish keep dying one by one

[mrseigel]

I am at a loss because over the past 1.5 months, all my fish have slowly died. I was stupid and put shipped fish in the display tank without properly quarantining. I tried medicating with paracleanse, focus, and kanaplex. The treatment seems to delay the death. But after administering the recommended dose, the fish start dying again once I remove the medication. I've lost about 20 fish including my betta. The interesting pattern is that they are dying one by one. It is not a sudden mass dyoff. Some of the fish have had red streaks of blood by their fins and will swim awkwardly before dying. My betta appeared to have swollen gills. I repeatedly have checked my water parameters and they are always normal (pH = 7.6 ammonia = 0, nitrite = 0, nitrate = 10-30, Kh = 4, Gh = 8). My question at this point is what do I do? Do i need to scrap everything and clean the tank with hydrogen peroxide? Or is there any way to let the tank run for a while without adding fish to rid whatever is causing this? My cory catfish and mystery snails seem unaffected so far so they are still doing well.

 

[Colin_T]

Pictures and video of the fish so we can check them for disease?
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the screen and doesn't leave black bars on either end.
You can upload videos to YouTube and then copy & paste the link here.

How long has the tank been set up for?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do water changes?

Do you dechlorinate the new water before adding it to the tank?
Do you have chlorine or chloramine in the tap water?
Do you have buckets and hoses specifically for the fish tank?

What sort of filter is on the tank?
What filter media/ materials are in the filter?
How often and how do you clean the filter?

What fish do you have?
What fish did you add?
What fish have died?