It does look like HITH, and it's good water changes cleared it up. I 2nd the suggestion above - large, frequent water changes and a sandy or bare bottom that makes it easy to suction waste off the surface. I actually recently re-did a Discus tank like this due to similar issues. Even with 4 50% water changes a week (in a 210 gallon tank) the substrate (planted aquarium substrate) was just holding on to too much waste for fish that benefit from really clean water.
