My marine tank , how do i keep it clean.?

My marine tank has only one porcupine puffer in it, its a coffee table tank so its plenty big enough for him / her, my problem is it only stays looking nice and clean for a few days after i clean it out....within the week its got algee on the gl and the gravel / ornaments / dead coral is covered in brown algee which i then have to take out and scrub clean again...this is doing my head in because its weekly, my levels in my tank are fine and i have had puff for a yr now and he / she is doing fine too, this has only been happening for say last few months and nothing has changed in the tank..i do a water change every sunday of 25%..does anyone have serious, good advice on what i can do...?? My only thoughts are to totally drain down the tank really scrub all the gravel ( which has not been done cos im unsure of totally empty the tank ) and put all back in clean, i will keep as much of the original water as poss to put back in....what do marine tank keepers think....??