I've never used Dr. Tim's, but every live bacteria I've ever used before has been clear in color. Is there an expiration date on the bottle?
Here is an Amazon review I found regarding the black color:
Customer Review
Amazon Customer
1.0 out of 5 starsDr Tim's RE-FRESH kill fish
March 28, 2019
Style: Re-FreshSize: 8 ozVerified Purchase
I bought Dr Tim's Aquatics RE-FRESH 8 oz and Eco-Balance 16 oz bottle for Saltwater from Amazon. None of them has Expiration date on bottle. I have 75 gallon tank. I put 20 ml of RE-FRESH in. The liquid is black with back residue on cap . Day 1: the fish are no longer active. Day 2: fish developed tiny bubbles on skin, the purple Grandma swim fast rubbing the skin on rock to scratch it. The protein skimmer is brown and smell rotten. By the end of day 2 the purple Grandma die. I changed 15 gallons of new salt water. and put in 20 ml of Bio-Balance with the hope to rescue other fish. Day 3: Cardinal die in the morning, and Blue Tang die in the evening. I am very disappointed. I email Dr Tim's website asking for help. 20 hours goes by no response. I am so disappointed. I am totally mourning my death fishes.