"Robert Flory" wrote in
gy.com:

Change water to remove nitrate in an unplanted tank. Most planted tanks
have low nitrogen, you change water to limit the odd crap that tends to
build up.

....and replenish trace elements.

~Empty

--
'You're not friends. You'll never be friends. You'll be in love till it
kills you both. You'll fight, and you'll shag, and you'll hate each other
till it makes you quiver, but you'll never be friends. Love isn't brains,
children, it's blood... blood screaming inside you to work its will. I may
be love's bitch, but at least I'm man enough to admit it.'
Spike