I have a double concrete laundry tub. It leaks. I see the crack from underneath the tub.

This is going to be a really tough one! Since this is a hairline crack, the material you fill it with has to have really low viscosity, not the least because it appears as if it has to actually fill the void going up against the gravity on capillary action alone - if I understood the situation right, the crack is (almost) invisible on the inside of the tub but you can see it underneath, so you have to work it from underneath.

I would try one of those marine grade low-viscosity epoxy resins like those by MAS Epoxies here (

)  and start in a corner (preferably one that's higher than the other), just apply a dab and see if it actually wets the sides of the crack and wants to go inside. If it does, apply some more going down the slope (if there's a slope), don't apply over the entire crack all at once. If you picked the slow curing hardener, you have about 20 minutes to work with it at room temperature, so use that time to start from one end of the crack and pace your dabs to go slow but still arrive at the other end well before the resin stops flowing, so the resin still has some time to penetrate further until it starts setting.  

Make sure the crack is dry before you start, by the way, to the degree that is possible - blow compressed air on it and/or avoid using the tub for a few days before the repair.

If Low-Vis does not penetrate, it's hopeless - there's nothing I can think of that can penetrate, permanently seal, adhere and reinforce and be water resistant like the Low-Vis epoxy. Perhaps you can still apply a fiberglass patch over the crack using the same epoxy (if it's not in a visible area) but that won't stop the crack from spreading, so not an ideal solution. 

There's one little issue I'm not sure how to avoid - I don't know if they sell the epoxy in minuscule amounts that you need for the fix. Perhaps they sell smaller sample packages - I'm just not sure. Alternatively, if you have a boat that needs some patching up (or have a friend with a boat), buy a regular size package and use it up within one year - that's the shelf life.