You should get neither. FIrst, spring water has many dissolved compounds, possibly unwanted metals like copper, that could damage any invertebrates. Second, refrigerator water isn't much better than tap water. The first link below shows the unreliability of tap water, which can be applied to spring water and refrigerator water.

RO/DI water itself shouldn't cost hundreds of dollars. While buying a filter is the best choice in the long run, you should be able to get it at your local fish store or even grocery store for around a dollar per gallon. This is where having a nano tank really helps. An alternative to RO/DI is distilled water, which is easily found at most grocery stores.

My second link is a suggestion for you since you have a nano tank. Nano-reef.com is a great forum for small tanks, and it has a lot of good information on livestock, equipment, and just many people who have successfully kept nanos.

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