Quite simple by purchasing a sponge filter or small hob
filter (remove media), add water from your tank, add your fish, and perform water changes every day (around 25%-50%). You will be kept on your toes during treatments so I warn it is not for everybody, but can be a successful option. I know you really want this yasha, but there will be more as I assure you it is not the last one to be sold
Patience in timing
If you do not purchase the yasha you can cycle a 10g and place the firefish in it afterwards for treatment.