Fish Tank (Water) Question?

I have 7 minnows and their water gets really dirty really quick well it got better when I took them out of a 1cup size bowl to a 1gallon size bowl anyways I have to change the water so often that its costing me to much in water the cheapest water I can buy is 89cents a gallon. And I change it every other day recently Walgreen's had a 39cents per gallon sale but I only had 2 gallons left(figures) I have tried the only changing half the water thing ,but the water still looks dirty....so I was wondering if I could use tap water with NovAqua (which im using in the bottled water) into it it takes chlorine and al kinds of stuff out and makes their skin healthier and prevents some kin of slim getting on the scales. Also I have been boiling my gravel when I change the bowls water I also put the bowl in the tub and run scaling hot water on the then wipe them out the a Towel....I never use any chemicals because im scared it will hurt the fish. Oh I have a huge 30gallon bucket in my back yard...if can use tap water can I fill it up with water put however many drops I need to put in and let the minnows live out side (i caught them in the canal down the street). Please share anything you can with me. Thank You