Hi Andy,
You need to do weekly waterchanges. (replace 1/3 of the tank water each week)
I do not use any contraptions for this. ( 1x 2 gallon plastic bucket & a lrg. plastic measuring jug)
Take out about 1/3 of your aquarium water and replace it with the same from the tap. (going by your post you are using tapwater and not treated water like RO) boil your kettle and mix it with the water you have in your bucket, try to get it as near as you can to the aquarium temperature, then add a spoonful of Stresscoat per 2 gallon bucket. and your done.
I have been using this method for over 25 yrs and never lost fish by it yet. Previous to Stresscoat I used Tetra Aquasafe but I haven't seen it around lately.
Regards
BigC