Hi Lisa67 and Welcome to our freshwater beginners section!
You are in good hands up there with Katty and karin. Sorry to hear of your losses and your situation with the Tiger Barb. Even with those of us who had previous periods in the hobby, these things do happen, we see hundreds of them in our cases here in the beginner section.
In my opinion, the key mistake you are making is the one of having the shops do your water testing. This simply does not work well. If you want to become a serious hobbyist (and a big cichlid tank would put you in that category) then you first need to truly learn about biofilters and understand them in a hands-on way. To have the learning experience (and to effectively follow the cycling of your filter) you need to have a liquid-reagent based test kit. Most of us like and use the API Freshwater Master Test Kit (found online if not in your local stores.)
One of the things that's fun about our forum is reading other people's threads and joining in the discussions. Please take a look at Hoppo's thread that was posted near yours. I wrote a comment to member Hoppo explaining the basic story of the "biofilter" and of "cycling" (the name we put on the process of growing the two specific species of beneficial bacteria to make a working biofilter.)
The three articles in our Beginners Resource Center that are key in your situation are The Nitrogen Cycle, The Fishless Cycle and The Fish-In Cycle. You have three possible paths to take: Fishless Cycling with household ammonia (this will give the best learning experience), a Fishless Cycle with fishfood or shrimp/prawns (this is a more difficult learning experience and many give up in frustration, but it can be used to achieve a fishless cycle) and a Fish-In cycle with the Barb (or more) (this will leave you with less hands-on understanding of biofilters in my opinion but it may be the only option open to you if you cannot re-home your fish back to a store or other hobbyist.)
You are, of course, currently in a Fish-In Cycle and as such, your concern -must- of course be the welfare of the fish. 95% of that concern is about ammonia and nitrite (as you'll read in my other post) and of course a little of it is about a good water temperature and a tank cover so the barb can't jump out and minor things like that. As mentioned, altering pH is not a good idea at all, we don't want other chemicals in the tank and a stable pH is better than a particular ph number.
Good luck with your decisions. If it were me I'd try to have the members help me find the right kind of ammonia (depends on where you live) and I'd try to rehome the Barb. Part of that opinion is because tigers cause such limited community stocking options (they are very aggressive and need a minimum group number and limit what the other species can be.) But no matter what you decide there are people here who can help you.
~~waterdrop~~
