Okay, I have a couple more questions for you:
1. What are the water conditions of the tank (Salinity, pH, nitrates/nitrite, phosphorus)?
2. How large is the tank and what do you have in there for a clean-up crew?
I used tap water when starting up my tank and didn't have a problem with it (that's certainly not to say you can't). I've also seen some amazing growth in a tank that was only given tap water. It could be your problem, but before you run out and start dropping exorbitant amounts of money.
Good Lord, that guy was trying to take you! Occasionally, I do 20-30 gallon water changes in a system that has about 100 gallons. At $10 for 4.4g, that would cost roughly $50! Get yourself a RO/DI, you'll save money (esp. if you get it used from someone here and just replace the filters).
Even before buying an RO/DI, it could be something as simple as a build-up of nutrients.
Eric