I would recomend the phosphate sponge, and instead of a 25% water change, do 10% once a week. this will take a while, but in the long run in more beneficial. As phosphates are not really deadly to a tank, i would not advise large water change. I am not an advocate of large water changes, except to get toxins out. patience is needed here, but in about a month, you should see a dramatic difference. Also, try to find the source of the phosphates- check the label of the foods you use, etc. test your water, even though you are using RO/DI before you put it in... before and after you add salt. Could even be the bucket you mix your water in--- ever clean it with soap and not get it totally clean? I used to mix my water downstairs, and realized that as it was next to our 'slop' sink, when my wife did laundry, small amounts of soap from the washing machine draining into the sink was splashing into the water! soap is high in phosphates....
with smaller water changes, you will be leaving a lot of the beneficial bacteria in, and if you are keeping your calcium and alk/PH in the right range, your tank
will be better for it, as coraline algae should slowly take over your green algae.
Just my 2 cents-- this is what i did, and saw more of a long term difference than in doing a large water change.