Yes, marine and fresh are just totally different worlds in many aspects of the water. Over the ocean the sun blazes down, unchecked/unshaded for much of the time and the water becomes loaded algae and microorganisms and of course high salt content thus creating an environment where "filterers" like invertebrates and mollusks can thrive. Many fresh environments are totally different, shaded in forests and clear, with a lack of the sorts of things filterers could use and with a mineral content that varies widely in different geographic regions. For different species, their region is their home and water and someone elses region and water might not be tolerated by the bodily regulation systems their species has evolved with.
Dwarf puffers like to have enough mineral content to put them at neutral (pH=7.0) or higher, whereas otos we know like things about neutral or a little softer (breeders are known to use pH=6.8 for otos.) So to have nice water for these two to live together you'd keep in the back of your mind that the oto won't really like it going to hard and the puffers won't really like it going to soft! But neither are known to be overly sensitive that I can tell and so the real issue is stability, as with most tropical freshwater fish. If you keep your hardness and pH reasonably stable (meaning that you don't use chemicals or other means to cause their pH/hardness to swing whole degrees in short periods of time) then they both should be fine. The good beginner maintenance habits we try to teach here help to establish these kinds of conditions in the aquarium. By using your given tap water and then performing regular weekly gravel-clean-water-changes you are maintaining your aquarium parameters closely with your tap water that will keep coming in as replacement water. By keeping the maintenance regular you are avoiding the pitfall of allowing minerals to build your hardness upward away from the original tap level (when water evaporates, it doesn't take the minerals with it!)
OK, so what test kits do we freshwater people use? Well, first off always make sure they were made for freshwater. A marine test kit may have some of the same reagents for tests but the color match cards will be different! Most of us like and use the API Freshwater Master Test Kit, but that is because of a number of details and that it is middle of the road and reasonably priced. The gold standard that's a little more trouble to use are the individual Salifert test kits. The Nutrafin kits are also used as a general kit when the API ones are more difficult to find. Most, if not all of these folks, and TetraTest also, make GH/KH kits to measure general hardness and carbonate hardness. If you want to go beyond pH, then this is the next step. pH does not have to go in step with hardness but often it does. What this means is that if you find your tap water happens to be pretty close to neutral then you might decide to deduce that your hardness is not going to be a problem and just not spend the money on hardness kits.
~~waterdrop~~
edit,ps: as you may know, otos need a ton of algae to survive and one trick to feed them that I've heard is to keep a covered (for bugs) clear container in the sun in you backyard with 3 or 5 rocks that fit a spot in your aquarium. The idea is to get all the rocks covered in algae from the backyard sun and then rotate one at a time in to the tank where the oto can munch (or otos plural since they like company, but they'll survive with less company in a small tank with good food.)
I will probably remember later.
