All those feeder blocks do is cloud the water and increase ammonia in the tank. They are especially bad for small tanks.
If you go on vacation again for a week to 2 weeks, just don't feed the fish. That's better than the feeder blocks and most fish can go weeks without food.
Do another small water change and recheck the ammonia after a few hours.
How long has the tank been setup? I agree with Julie...having no nitrates looks like the tank is uncycled, but if you checked it right after doing a water change, that would affect the results.