Water changes
It does depend 
For the fish themselves, accumulation of nitrates should indicate when a water change is required for the fishs’ sake. For the plants, that can be an entirely different story! There are many approaches to fertilizing and managing a planted tank. Some of them, in particular for high light setups, include large water changes in order to keep elements from accumulating due to heavy fertilization. The following article is a perfect example of one of these methods: http://www.aquatic-plants.org/articles/est_index/est_index1.html
Water changes also help bring in trace elements so generally speaking the more the merrier (assuming your tap water is not problematic). I personally do 50% weekly on highly lit tanks as in the article above. I do 50% monthly on lower lit tanks and have gone as far as 50% every 4-5 months on well balanced/low light setups, although smaller more frequent changes would have been preferred had I better access to the tank itself.
As for your tank, 20% weekly should be fine, but the only way to know if larger changes will improve on anything is to try them for a couple months. Just make sure you don’t make drastic changes such as jumping from 20% to 50% all of a sudden as your fish are not used to it. Try changing 15-20 gallons for a month or two and monitor any changes in the plant’s health.
Hope that helps Giancarlo Podio, LMD
Category: Philippines Internet Koi Society
