Sounds more like nitrites or something spooked them. Check your nitrites (note the "i"). Anything over zero is a problem.
When you say your nitrates are "slightly elevated", what do you mean? And are you sure about the spelling? It makes a vast difference.
In the meantime do a 20% water change and I strongly suggest you don't go near pH Down with a 10 foot pole - that stuff is potential lethal in the hands of a beginner (or anyone, actually). pH swings kill fish very effectively, far worse than high pH.
That said, your pH is a little too high so it would be a good idea to get some bogwood and add it to your tank (boil it first or soak it for a week so it doesn't stain your water too badly). Bogwood will slowly and naturally lower your pH without any shocking happening and will look nice against the tiger barbs.