We will be closed for 10 days, and the fish will not be fed.

I work at a place, which has apx a 30 gallon tank with 2 large koi goldfish and 4 smaller ones.  We will be closed for 10 days, and the fish will not be fed.  Will this be harmful to them, if we feed them extra a few days beforehand?

It is possible to buy a product which dissolves slowly over a period of time like this one in our shop
Tetra Vacation Gel Feeder Block 14 days 

This would be a better choice than simply not feeding the fish...