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Maybe the waste is a good thing? 
I would say at least 3-4 inches. If you wanted to use amano as an example his tanks seem to always have deep substrates. Sometimes it looks like 10 inches or more at the back of the tank. A deeper substrate helps to create stratifying layers of aerobic and anaerobic bacteria. Anaerobic bacterial growth can be good for plants because it creates a reducing environment deep in the substrate. Whereas more aerobic bacteria grow in the upper layers. I personally use 4-8", and have never had sulphur bubbles from the bottom even using a mix of peat and onyx sand.
If you do decide to do 1.5" you could use the sloping method for a deeper substrate in the back.
I would say at least 3-4 inches. If you wanted to use amano as an example his tanks seem to always have deep substrates. Sometimes it looks like 10 inches or more at the back of the tank. A deeper substrate helps to create stratifying layers of aerobic and anaerobic bacteria. Anaerobic bacterial growth can be good for plants because it creates a reducing environment deep in the substrate. Whereas more aerobic bacteria grow in the upper layers. I personally use 4-8", and have never had sulphur bubbles from the bottom even using a mix of peat and onyx sand.
If you do decide to do 1.5" you could use the sloping method for a deeper substrate in the back.