I'm so sorry. I too am new at this, and have lost 10 fish in my attempts to find the perfect water and chemical balance. I even named them all, and that makes it even more personal. In an apartment that doesn't allow any 4-legged pets, these are my "babies". Even if you only glance at the aquarium now and then, you can form a strong attachment to your fish.
My only tips would be to keep doing whatever you do that keeps the majority of the fish thriving. For example, for me, since I have VERY high chlorinated and hard water, I USED to by Betta Water, sold in pet stores, and add whatever chemicals I needed to that. It got reeeeeally costly. Now, I make my own "fish water" in a large bucket, then spoon it, with a funnel, into bottles, that I store by my fish tank, for quick water changes. After much trial and error, I've learned the EXACT safe amount of mineral salt, water conditioner and PHup. I boil water for 5 mins, pour it all in a large bucket that holds about 3 1/2 gallons, full. Then add all the above ingredients, stir it, then scoop it into small drinking bottles, for later use (can't use right away, because the temperature would be way too high).
Your tap water may have totally different levels than mine, so use tester kit/strips to find out what needs adding. I've also lost many a fish, because I remembered to cycle the water, but forgot to clean the filter parts and change the filter. Oops!
Anyway, since I've discovered exactly what to add to my water, and make the "fish water" bottles up ahead of time, I haven't lost any other fish. They're all doing great.
Good luck and again, sorry for your loss.
P.S.- Make sure ALL guests and residents know that the bottled water, stored by the aquarium is NOT for human consumption, but for quick water changes.
