Correct, topping off with RO does nothing to mitigate the build-up of dissolved wastes. In SW, one of the functions of a protein skimmer is to remove those wastes without having to remove all of the water as well. Unfortunately, skimmers aren't as effective in freshwater. With freshwater, topping off with RO means that you aren't adding the 'base' TDS to your tank. For example, if you have 2 ppm zinc in your tap water, with each top off you add more zinc, but nothing is removing it. Eventually, your tank will have substantially more than the 2 ppm zinc coming from your tap. Using RO means the TDS build up will be limited to what comes from the biological processes in your tank.
You can get a TDS meter for under $15, well worth it.
Shane Fender of Planet Catfish does nearly 95% water changes regularly. Drains, then very slowly refills. It's a matter of preference...or, the convergence of convenience and fish survival. I like doing weekly water changes because it means that I spend the time looking at the tanks, tidying plants up, observing fish, etc. Do the fish benefit from that? Probably. Would they benefit MORE from daily water changes? Probably, but not without a lot of work to set-up a system that was more automated to control temperature and keep the water level consistent (bubble nesters do NOT like it when you suddenly drop the water level so their eggs are sitting on top of a plant). Big water changes ARE more effective than multiple small water changes, and you can prove this quite easily. You need 2 gallon jars. Fill both with water and the same amount of dye, enough to be darkly colored. In jar #1, remove half, replace with clean water. In jar #2, remove one cup, replace with one cup of clean water. Repeat for a total of 8 cups. Compare the results. Jar # 1 will be lighter colored because you removed half of the waste all at once. Removing it one cup at a time removes less with each cup because each clean cup put in mixes, meaning the next cup removed has more dilute waste. You replace half of the water, but don't remove half of the waste. If you spread the 10% water changes out over several days, the net is even lower because the fish continue to create more waste each day. (You can mimic this in the experiment above by adding another drop of dye to the jar after removing each cup).
If I have time, I'll run through this and test the water with the TDS at each stage. Might use something other than food coloring, like salt or sugar.
