Personally, if I had 1 fighter, 2 guppies and 1 platy in a 40 litre tank with snails I would do a total clean out of the tank. Take everything out, chuck the water (just rinse the filter out in it), put bleach in the tank and wash the substrate as well. Throw the plants or rinse with potasium permanganate, refill the tank and replace the fish. Fighters are used to massive water changes and I can't see the guppies or the platy not being able to cope with it. Regular water changes and there will not be a spike.
Cheaper and easier than finding another solution (but thats my opinion for what its worth )