Increasing tank capacity

I have a medium sized tank which is very underloaded with just three
fish. It's been running like this for about a month now and everything
seems to have settled down and got comfortable. I started out with a
really cheap internal filter that blew lots of bubbles but didn't seem
to do a whole lot of filtering. When I pulled it to bits to see how it
worked I just couldn't see how it was expected to do anything but the
most primitive filtering. I guess you get what you pay for, and it was
cheap. Recently I installed a Hagen Stingray internal filter. I keep it
running all the time and the filtering and circulation it provides is
wonderful. My question is this - does increasing the filtering increase
the practical capacity of the tank? In others words can I carry more
fish comfortably? Obviously there would be a practical limit to all
this, but within reasonable limits would this be a correct assumption?
The Hagen has plastic foam, for biological filtering I presume when the
bacteria get established, and two activated carbon cartridges. Thanks
for any comments - I really am a little new to all this.

--
Peter in New Zealand.
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