Use of RO, DI (Distilled) water in Aquariums or Betta Tanks;
Unless you are an advanced fish keeper (which includes having GH and KH Test Kits/Strips) as well as some basic knowledge or aptitude of chemistry; generally the use of RO or DI water in freshwater aquariums should be restricted to blending with tap or well water so as to "Cut" the water resulting in lower buffers and hardness of aquarium for use with Amazon River, Southeast Asia (such as Bettas) or similar fish.
Generally I start with 25% to 50% and work up from this over time (if necessary). The reason is that RO and similar water is NOT properly mineralized for correct osmoregulation with essential minerals such as calcium nor is there any carbonate buffers to maintain a stable pH which the lack there of would result in a roller coaster pH in the aquarium, often with disastrous results.
RO/distilled can be reconstituted with some combination of products like ReMineral Plus, RO Right, Alkaline Buffer, etc, but for someone not particularly experienced with water chemistry, it is MUCH easier just to use a different water source that already has mineral content. Also cheaper.