OK, I would highly recommend reducing the feedings to once a day or every other day. I would ditch the pellets and opt for frozen foods, and you can use the pellets as a weekly treat. Frozen food is more nutritious than the pellets. I think what may have happened is during the water change some uneaten food may have been stirred up causing the water quality issues. This tank is just 12 gallons and any shifts at all can be very detrimental to the inhabitants, since there is a smaller water volume.