Do I have to use the saline solution they sell at the pharmacy, or can I just use tap water?
Can you?
Sure, you can also store them in sulfuric acid.
the proper question is Should you?
IMHO as a contact wearer, probably not.
In an emergency, yes. But I wouldn’t make a practice of it. IMO it is marginally better than sleeping with contacts in – unless you have the kind that are supposed to be worn 24/7. Your eyes need a bit of exposure to oxygen and it is important to give them a bit of a rest.
Tap water obviously does not have the right salts in it. It can contain micro organisms that your eyes aren’t used to. The lenses will absorb the water which because of its different properties will change the shapeof the lens, and the surface properties of the lens will likewise alter.
All of this is rectified after the lens has been in natural tear fluid for a while – assuming of course that you don’t get an infection. But it does take a bit of time and puts some stress on the eye as a result.
Only in an emergency. I am not sure if you have tried it yet or not, but putting in contacts that have been stored in water hurts like a “motha.” Also, a lot of the lens storing solutions have disinfecting agents in them as well I believe, so it is worth using them to avoid infection.
Rigid contact lenses store better in water than soft lenses do. Even among soft lenses, some brands or types do better than others.
However, it’s never recommended. Some of the bacteria that are common in bathroom areas can cause pretty serious eye problems - to the point that my ophthalmologist always recommended replacing the contact lens case every three to six months. Any kind of eye infection is pretty nasty and you don’t want to take the risk if you have any other choices.
You can, but you’ll always remember to bring contact solution with you after the first time you try putting them back in.
Definitely not recommended. Without the right salt balance, my contacts shriveled up…of course, I couldn’t see my contacts since I didn’t have my contacts in (my prescription was in the -12 range…with astigmatism).
Ouch, it hurts just remembering trying to put them in.
Thank God for eye surgery 
I did once… YOWSA! Then again, my eyes already hurt because they were sensitive to the solution, the tap water wasn’t any better… so I switched to dailies.
Tap water isnt sterile and has chlorine and other irritants in it. You should always use a sterile solution unless your idea of good time involves an eye infection.
Considering tap water won’t do anything to clean the contacts, which is what the solutions the pharmacy sells does, it seem storing them in tap water is a great way to get an eye infection.
If you like amoebae in your eyes, you can.
Yes, yes it does. You wouldn’t think so, but it’s really unpleasant. Also, if your water isn’t perfecly pure, you’re just putting extra stuff on your lenses that can build up (like that white stuff you get from hard water).
ETA: I usually* wear the kind of lenses you can wear 24/7 for a month straight, then throw them out, so I don’t go through solution very fast, anyway, but the Equate/store brands work as well IME as the more expensive name brands for me.
*I also have several different colors, but my colored contacts are two-week disposables, and I can’t wear them to bed.