I was using Coconut oil to fry up tostada shells and I was wondering if its safe to reuse the oil I used the night before to cook up more shells the next day?
For most oils, you can re-use them several times, if you're OK with (or want) the flavors the oil takes on from the food cooked in it.
The thing to watch is the smoke point. That's when the oil starts sending off whisps of smoke. For coconut oil, that's 350F. As oil is re-used, that smoke point starts to drop until it's no longer able to stay hot enough to properly fry food.
We're dealing with an oil that has a relatively low smoke point in the first place at 350F, as opposed to 450F for peanut oil, and proper frying needs to be pretty close to 350F in order to have the moisture in the food hold out the oil from rushing into the food like a sponge. That, unfortunately means that coconut oil isn't a great candidate for re-use.
Please see this question: Frying Oil Reuse. It's not explicitly about coconut oil, but I don't know why there would be any difference.
I reuse refined coconut several times. I run it through a very small strainer and pop it in the fridge if I am not going to use it again in the next day or so. I use it mainly on veggies I put panko on the outside. The cooked veggies do not leave a very strong flavor in the oil. I cook them low and slow.