good luck. hope you can get a bigger tank ... that may be your long term solution.
In the meantime - about the % of water changes - look at it like this: all of their waste and food waste is in their environment. the cleaner the water the more pleasant and healthy
as long as you're doing a water change, why not just change a little more? nothing hurt and I'm sure your fishies - that you obviously care about very much - would appreciate it.
again - good luck. I hope they stay mellow for you