Ziggy,

I agree totally with the reply from sayhy2Mark33......

A reactor is one of the best ways to go and can be kept outside or underneath the tank.

Please read this article and see if it helps.

Dual venturi DIY External CO2 reactor

There are many other threads/posts on this site that will help you.

That said, how big is your tank? That will dictate the size/type of equipment, no?