After having a happy healthy tank for nearly a year, I'm starting to lose it all. I'm getting mixed opinions from my local aquatic stores and I would like the definitive answer please. Here are the facts. Tank is 275 litres. External All Pond Solutions EFX filter (2000 lph) filled with Biohome and Alphagrog media with a pre filter. Columbo Nutri base topped with an inert black gravel. Rustic slate and red moor wood. Lots of Crypts, Java Fern and Mosses. Gouramies, lampeyes, Cardinals, Marble Hatchets, Glass catfish, Halfbeaks. Ammano and Cherry shrimp. My water quality is pretty much always ammonia 0 nitrite o nitrate 5 - 10ppm ph 7.4 - 7.6 Temp 26.6 - 27c I do 10 - 20 % water change every 1 - 2 weeks. In November I introduced a couple of new fish. A gold gouramie, male dwarf and 2 sunset honey gouramie. I don't want to jump to any conclusions or point the blame at these new fish. But, a few weeks after this the male dwarf showed signs of whitespot which I treated with Nt Labs whitespot treatment. I had to treat twice as the first week of dosing did nothing. Ulcers then started appearing on the male dwarf and on another gouramie, as well as losing its feeelers. So after some advice from Nt labs ulcer treatment was added. They also suggested raising my KH as it was zero, so I bought some of their KH up and added it to my tank. Its now 4 but the pH rose to 7.8 The male died as well as the gold gouramie. Whilst this was going on the lampeyes started dying ( 6 in total ) and the cardinals started swimming at a 45 degree angle with laboured breathing then died. The Gouramies as well as the ulcers had lost their colour in their flanks. ( please keep reading if at this point you have diagnosed the fish disease, that I don't want it to be ) After this string of deaths the tank settled down and was fine for 4 weeks. At the beginning of last week another gouramie showed signs of ulcers, I took it out the tank and disposed of it, and now two hatchets have died. I must also add that some of my plants started dying too. The mosses browned a little, some of my crypts are also losing leaves and a lot of the java ferns browned and died. A local store has suggested not jumping to any disease conclusions as yet and to get the water chemistry sorted out first. So, I am trying to get my ph down to 7.2. My house supply out of the tap is 7.2. It looks like the KH up powder has also increased my ph. Over the past week I have done a couple of 30 - 40 % water changes and added Waterlife pH to 7.2 to the tank during the water change. KH is now 3 but ph is still 7.8.
What is the kH of your tap water? It may be low, but possibly enough if you up your water changes to 25% weekly or a tad more rather than adding a buffer. That way your pH won't rise as much. Though to be honest, the pH is unlikely to be an issue if your water is soft, which is the more important factor for soft water fish.
As to the disease, that's a separate problem, and I would suggest treating with Esha 2000 to try and cure that. What water conditioner do you use? Some can actually make medications less effective so that may be why treatment has been less than effective so far.