Let me give you some background. it is a 9" tank, there are 3 zebra and 2 Gourami. however, after i put a plant which is attached to a wood, the water becomes acidic (ph7 to ph 5.5/6). a few weeks later, a gourami becomes sick and died. After that, everything seems ok except that the water is still acidic, then one month later, this Gourami gets sick also. I changed half of the water and add some salts as suggested by the pet shop people. but still not much improvement. ( i do not use water conditioner but i will leave the tap water overnight before i use it)
how could i help the fish?