I can't seem to keep Corys alive

[Herpin Man]

I have a 35 hex tank that has been set up for about a year. It's planted, I have both undergravel filtration and a hang off the back. I have gouramis, black tetras, and lemon tetras. Zero ammonia or nitrites, regular water changes, and temps in the mid 70's. All of the fish are doing great, other than corys. I've gone through about a dozen of them.
What could I be overlooking?

 

[Byron]

We need more data. Can you post the GH and pH of the water (the source water and the tank water if different). What is the substrate material? What cory species? And are you diligent about cleaning the substrate under the filter plates?

 

[The Lumpfish Guy]

I am assuming you mean a 35gal? (from the UK so I work in Litres)
Numbers of fish would be helpful as well?

 

[PheonixKingZ]

What are your water parameters?

How long has the tank been setup?

We cannot accurately help you without all of your information. Good luck!

 

[Colin_T]

I am assuming you mean a 35gal? (from the UK so I work in Litres)

WOO HOO... Litres, Litres, Litres, Litres
I'm from Australia where we haven't used gallons since the 60s. The rest of you imperial users need to get up to date with the rest of the world

 

[The Lumpfish Guy]

WOO HOO... Litres, Litres, Litres, Litres
I'm from Australia where we haven't used gallons since the 60s. The rest of you imperial uses need to get up to date with the rest of the world

I would be happier if we didn't stop half way and still use, miles (+MPH), feet and inches, pints (although I do like this one, asking for a pint sounds better than litre) ect.
It's trying to figure out who means US gal and who means Imperial Gal (or UK gal, for the older UK residents) that gets me. If i remember rightly there is a 5L difference between the two!

 

[Colin_T]

US gallon has 3.785 litres
UK gallon has 4.5 litres.
1 pint = 600ml
1000ml in 1 litre.

Imagine going to the pub and saying gimme 600ml of beer

 

[The Lumpfish Guy]

US gallon has 3.785 litres
UK gallon has 4.5 litres.
1 pint = 600ml
1000ml in 1 litre.

Imagine going to the pub and saying gimme 600ml of beer

No idea where I got the 5litre difference from...

Yeah although I would rather hope they were served in Litres...

 

[PheonixKingZ]

I’ve always learned it in gallons.