When I notice the ink going down in my pen, that means I've been practicing a lot, which makes me happy.
When it looks like the ink in my pen is getting low, that means I've been writing a lot, which makes me happy.
この場合、次のような言い方ができますよ。
ーWhen I notice the ink going down in my pen, that means I've been practicing a lot, which makes me happy.
「ペンのインクが減っていくのに気がつくと、たくさん書いているということだから嬉しい」
ーWhen it looks like the ink in my pen is getting low, it means I've been writing a lot, which makes me happy.
「ペンのインクが減っているようだと、たくさん書いているということだから嬉しい」
インクが減ることは to go down や to get low を使って表現すると良いでしょう。
ご参考まで！
カナダ 
