Yup, RO water is very soft and needs to be buffered to restore it to normal pH. If you just need to adjust the pH to match your aquarium conditions so that you can perform a freshwater dip safely, then good old baking soda or sodium bicarbonate should suffice for adjusting the pH upwards.
However, in my experience, the best way to stabilize your pH in your aquarium at the proper level is to gradually adjust it upwards as usual, and then use a dual-phase or 2-part Calcium Buffer System periodically thereafter. This type of buffer has two parts — an alkalinity component and a calcium component — that simultaneously adjust the carbonate hardness of the aquarium as well as the calcium level, which is very beneficial for seahorses
To adjust your pH to the proper range (8.1-8.4) initially, just obtain one of the commercially made products designed to adjust the pH upwards in saltwater aquariums and use it according to the instructions. Such a product should be available from any good LFS that handles marine fishes and invertebrates; they typically include sodium bicarbonate as their primary active ingredient and are often marketed under names such as "pH Up" or something similar. Just be patient when you are adjusting the pH and don’t add too much of the product too soon. Very often your pH won’t budge at all the first several times that you add the product according to directions. That’s perfectly normal, so don’t be discouraged if your pH stays at 7.8 even though you’ve added several doses of the product you obtained to raise the pH. Don’t don’t be tempted to add more of it or to add it more often than specified in the instructions. The product must first overcome the natural buffering ability of the saltwater in your aquarium before I can change the pH level significantly. It’s like performing a titration — typically, you add several doses and your pH doesn’t budge at all, but then the very next dose you add may change the pH dramatically. Since you never know when that critical point will be reached, remain patient and continue to carefully add more of the product as directed until the pH does start to change, and then adjust it to the desired level as gradually as possible.
Once the pH has been adjusted to the proper level of 8.2-8.4, you then add the alkalinity component of the 2-part buffer system. Next you wait a couple of minutes and add the calcium component of the 2-part buffer system. Your pH should remain stable at that pH thereafter and this method also has the added benefit of keeping your calcium level in the proper range as well. For a typical seahorse tank, you can keep it stable at the desired pH by adding more of the 2-part Calcium Buffer System about once every week or two after you perform your usual water changes with the RO-mixed saltwater.
The 2-Part Calcium Buffer System that Marcie and some of our other members report works well with their seahorse tanks is labeled "ESV B-Ionic" on the bottles, but Sea Balance and many of the other brands do much the same thing. The alkalinity component of these two-part buffers maintains the carbonate hardness or KH in the aquarium, whereas the calcium component maintains the calcium levels in the proper range. Any good marine aquarium store will have a suitable product available for this.
Best of luck adjusting your pH and the eradicating those black wormlike parasites, Dave.