Collitchboy you are in good company when it comes to recently unemployed (and/or currently "unemployed") students. I agree that test kits are expensive and there are still a few of the less critical ones that I need acquire. That said, not testing and having a tank crash is much more expensive. Also, there is always LFS's. Most will test your water for you.
I have also been using the Culligan water from Walmart now for quite a while. It doesn't seem to have any serious problems, but I have heard some people say that at some places it is loaded with phosphates. If you're having problems with phosphates, this could potentially be a source of your problem (and therefore water changes aren't going to help you out). I'd get the source water tested. Once you know it's clean, just make sure that the maintenance schedule on the Culligan machine is being filled out. They are supposed to test for TDS and do maintenance on it once a month.
Your tap water is also another potential source of nitrates and phosphates. Although, in very small amounts (i.e., just topping off) they won't matter as much as some of the treatments that some municipalities put into their water (e.g., chlorine). Another thing you need to watch for if you use tap water and plan to keep any inverts is copper. Both apartments I've lived in since I've started into saltwater have had significant amounts of copper in the water. When I first started I was using tap because I had always used it for freshwater. My snails and hermits kept slowly dropping dead after a few weeks in the tank and with no other water parameter problems (even the tank tested fine for copper because I was running carbon that was pulling it out). However, it was present in my source water and so every time I did a water change I was dumping more of it back in.
I'd get the fuge if for nothing else than the food it will provide for your tank.
Just my opinion... hope it helps.