250mg fine. You can use up to 500mg.
But like I said I would only use the antibiotic in isolation.
This is how you prepare the antibiotic food, But I forgotten the dosage.
Soak some pellets in the white of an egg.
Crush the tablet into a powder.
You just dab the soaked pellet into the antibiotic powder.
Then place in fridge to set.
If some of the antibiotic gets into the water than yes it will affect your filter bacteria.
I would buy a small hospital tank with equipment, there quite cheap.