The non-contact thermometer will be OK for getting an indication of THE SURFACE TEMPERATURES in the tank and the
bath water.
Bear in mind the type of thermometer you bought will set up with a very wide measuring spot and an unchangeable / untuneable emissivity setting which will be fine for some materials , but will make it very inaccurate at reading the surface temperatures of other materials who have surface emissivities very different to the general e = 0.9 commonly used.
You need to know the diameter of the measuring spot at any given distance from the "gun" and be sure to avoid holding the "gun" too far away when measuring else you risk averaging the temperature of the spot you want to measure and those of background surfaces which are likely to be colder.
Probe style digital thermometers with probes on the end of long wires are better as you can measure surface and fluid / air temperatures very accurately and need not worry about emissivity values.
I use these in all my vivs , several per viv :
http://www.ebay.com.au/itm/4x-LCD-Digital-Temperature-Thermometer-Fridge-Freezer-Aquarium-Fish-Tank-Reptile-/391259141696?hash=item5b18dcb640:g