Sorry when I meant 6 plants I meant 6 types of plants, I've got more than 6 plants in there right now, probably 10-15 and I'm planning on getting some more soon.
I'll see if they have Flourish and Aquarium Pharmaceutical’s Aquatic Plant Stimulant at Jansens. The prices for Potassium sulphate and Magnesium sulphate at 'Scientific Supplies' are $48 for 500g, which is the minimum amount you can purchase! I'm going to check out the prices at a hydroponics store, see if they are any cheaper (or if I can buy less).
Thanks for the advice Warren, you've been quite helpful.