What is the Ph of your water out of the Tap? Absolutley you need to improve your water quality but, you don't want to stress them with too large a jump in PH. But since you already did a 50% water change you should be pretty close now. You need to get some new test kits if your are over 6 months old and do some actual test so you REALLY know what you parameters are not just that you believe they are good. I'd be willing to bet that even with your water change your Nitrates are over 40ppm. If you have the tank space I'd move all the fish and filters and change all that water and clean that tank. Then fill declor age and heat the water for 24 hours then put the fish and filters back. JMHO
Scott