Since you have a small tank and are on a budget right now I would suggest the following:
0ppm Portable 50 GPD Reverse Osmosis Ro Di Filtration | eBay
I had the exact same one and it worked really well. It can hook right up to your garden hose or faucet and be out away when youre done. It wont need new filters for at least 6 months so you can get by with it for quite some time, probably longer since I dont think you'll be going thru much water for just a 40g.
A much better unit is of course available thru bulk reef supply but they are more money. I invested in the bigger unit after about a year of using the ebay one as I make several hundred gallons of water a month.
There is no reason cost should keep you from using ro
water which is one of the best things you can do for your tank.