10% - 15% bi-weekly water changes has little or no impact on water quality. I would do a deep cleaning of all substrate, filter and tank walls and increase water changes to 50% daily and add regular salt @ 2 TBS per 10g for 10 days until this clears up. Replace the % of salt being removed during water changes. I always age my water, but if you don't have the capacity to age 50% then try aging as much as possible and use tap for the rest. Nearly all fish can handle a 25% water change straight from tap, so if you use 15 gallons of aged water and 15 gallons tap (or less) you should be ok. No meds should be necessary. Good clean water should handle this but keep us up to date on its progress.
Overall bioload can produce these exact conditions http://forum.simplydiscus.com/showth...-Are-Important