Fluval Filter

[KittyKat]

Its not so much my stock its when i add them to the tank if i buy 4 i normally end up with one maybe 2 left.

To me, that indicates a problem with the aquarium or your stocking. A knifefish in your aquarium is a problem with the stocking. Going by the information you provided us with already (which does not include numbers and species), I suspect the 10 fish died because your aquarium is overstocked with unsuitable species, with the filter merely exposing the problem.

 

[dave1707]

In my tank i have 1 BGK, 2 gouramis 1 plec, 4 silver dollar, 3 golden danios, 2 zebra danios, 2 corys.

 

[KittyKat]

In my tank i have 1 BGK, 2 gouramis 1 plec, 4 silver dollar, 3 golden danios, 2 zebra danios, 2 corys.

…and what did you start with before the 10 died?

There are existing and potential problems with your stocking:
* 1 black ghost knifefish - too large, problems already pointed out
* 2 gouramis - species? most common ones are too aggressive to keep together in a tank that size (out of common ones, 1m 1f / 2f are ok if they are dwarf or honey gouramis)
* 1 plec - species? most grow too large for that tank
* 4 silver dollar - too large, schooling (should be kept in groups of 6+)
* 5 zebra danios - schooling (and personally, I prefer to see them in a 3+ ft tank)
* 2 corys - schooling
…as you can see, there are a number of problems, all of which contribute to the fish's base level of stress. Also, with those fish, the tank is overstocked, which is why it was not able to support them for a few hours without the filter running. With the other 10, the stocking levels would have been on the dangerous side of things. This also probably explains why fish die when you add new ones.