I have a cycled tank thats a 54 litre but its got 5 platy fry in it so cant move my adults there as will transfer the white spot. I also have cats and worry about where I would put the food box with fish in it without secure lid?? My filter is an external hang on the back of the tank style (Marina) so dont know how I would transfer sponge to a filter that was underwater in the box as it would be a different shape?? what filter would you recommend I put in the box ??? sorry for all the questions but it gets more confusing every post??

How big are the Platy fry? If they are big enough not to be eaten then as I assume you'll be returning the inverts to the shop I would do a couple of big water changes on the small tank to get the ammonia and nitrite right down and treat the fish where they are. Do diaily 25% water changes and adjust the med dosage to make up for what's removed during the changes and as soon as treatment is complete move the fish in with the fry.