I am answering to the best of my immediate abilities and time:
excessively high pH may well screw up fish (esp those like Gan's tetras which are wild caught). But high ammonia and nitrite will probably do worse..... is your tank heavily planted, set up for long, and the plants thriving???? If so, nitrogen compounds shouldn't be a big issue, unless you have had major deaths or melt-downs recently.
Are you sure your tetras are not suffering from fungus or other parasites???? Can i recommend you (and all other hobbyists) this book: Tropical Fishclopaedia by Mary Bailey and Peter Burgess click here, which i find very useful in diagnosing fish diseases, treatments, water maintenance and very sensible advice on fish-keeping. It's available in some good bookstores and is well worth its cost of about $30.
Unless your place's tapwater is at fault, you probably have some water hardening (coral chips, shells, dolomite etc) substance in your tank or filter.
I think Amano gives good advice in avoiding chemical acidulants to manipulate pH. If you can fix your tank's water using the standard KH/pH/Co2 table it should be much better and more predictable.
What do you mean by "But is ok to use Potassium Sulphate to raise the PH is required"? I catch no ball.
The only water softening/ph lower material i would use are peat-based. But local tap water is already very soft and slightly acidic to neutral. Unless your test kits are crappy (and many are - try borrowing a ph Test pen from a LFS or other instead), i can't say for sure why your pH is so screwed. But if it's something in your tank/filter, it's best to start afresh.
Changing water helps keep the mineral content of the tank from building up (as water evaporates), so changing the water (30-40%) every 2-3 weeks can help, but not if something else is fundamentally wrong.
I think cleaning filters every 6 months or so is sufficient. but check is your filter media might be the problem.
Lastly, nitrate will NOT be a problem if you have healthy plants.
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