you might want to make an auto water changer it simple to do and eliminated the weekly hassle my wife likes the tank much better now that no work has to go into in.
I simply drilled a 1/2 inch hole at the far end of my tank and ran tubbing to a drain. on the other end I ran 1/4 water line and attached a inline filter form low for $12 and a small section of 1/4 tubbing for the filter to a drip atmiter for the irrigation dept cost about $2.00 there are auto water changer caulator on the web that will tell you what size emitter you need to change out the volum you want. I change out 50% on a 300 gal tank. my arowana has grown form 7 inch to over a foot in about 3 month. I think my sucess this time is due to constanc water condition