Changing 6 gallons of water in a 29 gallon is not really too much. I would go ahead and do the basic EI schedule and see if things go OK. Ignore the water changes. I mean do them as needed to keep the frog mess cleaned up, but pretend you are not as far as fertilizing goes.
If you find after a couple of weeks that the plants are still demanding a bit more fertilizer, then up the dosing just a bit.