A big spot on my puffer (and it's not a spotted puffer!)

[hillegom]

Anything that dies in you tank will release potassium and nitrate and other chemicals as well, ammonia only being one. When you add food, your fish digest it and release wastes.
Ever wonder why your grass gets greener after your neighbours dog, uses your lawn as toilet?
In your aquarium, you want to get rid of these wastes. Water changes help. As well, nutrient export helps a lot too. One way is to put a light above your sump and grow cheato. Put the light on a reverse cycle of your display tank and that will also help stop fluctuating PH Between night and day.
You can put the cheato in the DT as well, but to me that is unsightly, so it goes into the sump.
Seeing as you have algae on your back wall, it is a simple thing, when you do water changes, to syphon the algea off the back wall. It will grow back, but you will be doing an export of nutrients.
JMO