水草情迷

here is the spec. of my tank.
tank size: 18" Length, 10" width, 10.5" height
water volume : 18" length, 10" width, 8" height (23L of water)
lightering : ada tube T8-15Wx2 (30W)
change water 1/3 each time, twice a week.
2215 canister (not with full power)
co2 : have tried 1 bubble/sec, 2 bubble/sec and 3 bubble/sec
water temp now : 25 degree

I have try everything as far as i Know. still not make the plants any better with bubbles.
do you need more details?

just one more thing is that some leaves are melting. so, i cut off the top of the plant, but still no bubbles come out.

oh, i see some bubbles coming out from the plants after turn off the canister for awhile. does it mean that the water quality has no problem?
why some plants are melting or change yellow color and some plant's leaves are facing downward?

just wondering why the canister is too large for my tank? actually, i don't see the water is running allover the tank when i turn it really small flow rate. Is it any better to use 雨林管?