[ray-the-pilot]

If you put 1 teaspoon full of a chemical in your tank, you put 1 teaspoon in your tank (that's easy).
If you put 1 teaspoon of a chemical into 500ml of water (this is about 100 teaspoons) then take one teaspoon of this solution and add it to your tank you've add:
1/100 x 1 teaspoon of the chemical. i.e. 0.01 teaspoons.
The remaining .99 teaspoons are still in the solution that you didn't use.

 

[ray-the-pilot]

I think you have a problem

I

How can 5ml be 1/8th teaspoon?

I have the same question. The last time I checked, 1 tsp = 5 ml.; so, 1/8 tsp = 0.625 ml.

I'd do a couple of 50% water changes on your tank and hope for the best.