55 gallon - 48x13x20
75 gallon - 48x18x20
The tanks are the same height, so any light that is high light on one is high light on the other. The only significant difference is that the substrate would not be as uniformly lighted on the 75 gallon tank. A very good way to light those tanks is with one 2 bulb T5NO light on the 55 gallon, and two 2 bulb T5NO lights on the 75 gallon, with the lights widely separated. That gives you good medium light for both tanks. Two bulb T5HO lights should be raised about 3-4 inches on either tank, to avoid having such high light that algae is your main concern instead of fish and plants. CO2, not Excel, is a very good idea with any of those light setups.
75 gallon - 48x18x20
The tanks are the same height, so any light that is high light on one is high light on the other. The only significant difference is that the substrate would not be as uniformly lighted on the 75 gallon tank. A very good way to light those tanks is with one 2 bulb T5NO light on the 55 gallon, and two 2 bulb T5NO lights on the 75 gallon, with the lights widely separated. That gives you good medium light for both tanks. Two bulb T5HO lights should be raised about 3-4 inches on either tank, to avoid having such high light that algae is your main concern instead of fish and plants. CO2, not Excel, is a very good idea with any of those light setups.
