Dead Kuhli Loaches and tetra help!!!

[Puffereon]

So in my 20 gallon tank I have 2 upside down catfish, 4 kuhlis (2 Dojo, 2 Black) and 3 tetras.

The black kuhli was found at the bottom of the tank, dead. The loach was covered in a white crusty film. When I took him out of the tank the crust started to come off in flakes. And the other black loach also died with some flakes. And also one of my tetras was swimming upside down and was covered in spots before it died.
Help me find out what this is before it spreads!!!

Thanks

 

[OrangeCones]

What are your readings for ammonia, nitrite and nitrate? Poor water quality can make fish susceptible to many diseases.

 

[Puffereon]

Everything is good but the nitrate is slightly high.

 

[OrangeCones]

Everything is good but the nitrate is slightly high.

What level is considered 'good' to you? What nitrate level is 'slightly high?'

 

[Puffereon]

What level is considered 'good' to you? What nitrate level is 'slightly high?'

I used the test strips and everything is in the safe zone and the nitrate is slightly out of the safe zone.

 

[exhumed07]

test strips are a no no. go with a liquid test kit. they are much more accurate. also it reads the exact parts per million, not just safe or unsafe.

 

[Zephyrah]

can almost guarantee its an ammonia problem. It kills fish very quickly. Definitely get the liquid test kit, even if you only get the one for ammonia instead of the whole kit. It will save you a ton of fish!!

 

[Zephyrah]

Amazon.com: API Ammonia Test Kit: Pet Supplies They sell these at all the pet stores and on amazon.com

 

[OrangeCones]

I would recommend Seachem's Ammonia Alert disk.