seriously... F8's are commonly found in FW in the wild. but they do better in a low end brackish environment (around 1.005sg) in aquaria. it helps keep disease at bay.
http://www.aquariumlife.net/profiles/tetraodotidae/green-spotted-puffer/100030.asp
i did 20% weekly WC on my puffer tank, and my fish where fine. i kept a GSP though, and the tank was a 30g, so the waste was probably less concentrated than it would get in your 10g. so i underestimated... sorry about that

i didnt overfeed though, even though most puffers will always beg for food.
i fed every day when he was < 1", then every 2 days till about 2", then every few days at 2"+. i only gave him food till his stomach was just a small bump. most people will feed and feed till it stops accepting food, which will take a while. i think this is where the exess waste comes from... rediculously overfeeding. i also took out any uneaten food.
a 50% WC in one go seems a bit too much though... if it was my tank, id do 25% every few days instead... less stressful for the fish.
chances are your puffer will tell you when its not happy anyway, as thier stomach will turn grey/black. i used to do a WC every time my puffers stomach turned grey. as well as the usual sunday WC.
also make sure you put loads of decor in there... a busy puffer is a happy puffer. if its going up and down the glass all day, its bored, and needs some mental stimulation. puffers are really intelligent, and can become bored very easily. try to rearrange its tank every once in a while to keep it amused.
make sure your 10g has a lid and all openings are sealed... thats why i dont have mine any more. they have good aim, mine got through a gap as wide as he was.