Is there any quick fix for this issue, I saw in the db it's int.

I can't think of a fix for this off of the top of my head, because there's not really anything I can think of to fix. Can you please help me understand a use case where quantities should not be whole numbers?

I think you shouldn't use quantity as you intended.
The shop is made to sell products, not increments of goods.

To solve your problem you could create a product (a 'virtual' Package) that contains 0.5kg of your goods.
Then the client can add multiple of this products to the basket.

I think Will is right here: I too can't think of a case where a client can only buy half of a product.

[quote=Reto Cossalter;1447]To solve your problem you could create a product (a 'virtual' Package) that contains 0.5kg of your goods.
Then the client can add multiple of this products to the basket.[/quote]

This is a fantastic suggestion/workaround. Thank you! :)