Nothing wrong with deionized water, but as soon as it hits your lines it will get plenty of ions in it. Don't expect it to stop your parts from shorting out in a leak, but it should be free of Ca and for the most part biological growth. RO water is technically not fully deionized I think (16Mohm or so); a real deionization system will be 18.2 MOhm and also typically will take measures to remove pyrogens, DOC, and RNAses and DNAses. Just spent $8000 on a good water purification system for my lab
