Fluval 304 on a 28g, how much flow?

I think that you can run that filter all out on that size tank. Most fish, even my little endlers and Angels can swim through the current w/o issues

I have a 304 on my 55G and I added a koralia 1 powerhead. I still could use more flow. If I had to do it over, I would either gotten a 404 instead or a koralia#2 instead, maybe both?

Run it full open.

On several 45 gallon tanks I have FLuval 404, Rena Filstar XP3 or Aquaclear 110 (HOB style) filters, and have added Koralia or other power heads. The 304 was too small for a 45, OK while it was understocked and the plants were small, but once things started getting big the force from the filter was not enough water movement in that tank.