I don't believe they would ruin your reservoir. However, you need to clean it thoroughly after every use.
The build up of smallest amounts of any substance, not cleaned, will breed bacteria and result in (first) small dark dots. Then gets much worse.
Even with the small dark spots, the first use of your bladder after that will result in one outcome. You will spend your next walk running for the bushes.
Joking aside, this could result in serious dehydration issues.
Don't ask how l know this