This is a good question really. The answer is CO2/carbon. The idea with limited water changes is that you are trying to minimize CO2 fluctuations. The plants can not adapt as quickly as algae to changing CO2 levels so algae will be more likely to get a stranglehold. The plants also play a role in the filtration in these tanks helping to minimize the need for water changes. However, with this kind of tank you can not have a high fishload because the waste in the water would accumulate quicker than the plants can use it.
In dosing excel you are also keeping the CO2 (really organic carbon) levels constant by dosing excel daily. So you are using excel for your carbon source. I think excel is out of the water after 24 hours (it doesn't last long!). As a result you can keep somewhat higher fish loads and do water changes, nutrient uptake and growth will be increased due to the excel. Excel will keep the CO2/carbon constant and also may prevent algae.
Hoppy is right, CO2 even DIY, or excel will help your plants in your 50 gallon setup, the above tries to explain why that is the case. In my opinion it is difficult to do the low light no water change tank without a decent substrate as a nutrient source. You really do need to pay attention and supplement the tank. Root tabs can help, but they really don't replace a good substrate. The reason why CO2 enriched tanks can do well even with plain gravel is because many use EI and dose the water column with everything the plant needs and the number one need is CO2/carbon, so you can take care of other needs. Light and carbon are the number #1 needs for the plant. With lower light carbon is still very important, you just don't need as much as your would with a higher light setup.