I started with a new 20 gallon tank in December. I didn't know anything about cycling - so I filled the tank with water, waited for the temp to get to 78 and bought 4 zebra danios and a beta, a week later I added 3 ballon belly mollies, another week went by and we added 2 placos, 2 neon gourami, and a couple guppies. I had done about a 20% water change everyweek for a month when the water turned green and everything started to die. The guy at the store knoew right away that I had left the light on 24/7. He suggested a 50% water change and treat with "no more alge" once a week for 3 weeks. Well everything except the danios died within the first week. I bought a 10 gallon set up - Filled it with warm water and got the danios out before they met ther demise. (so far so good - for them). I completely clean out the 20 gallon and started over. The tank has been running for 3 months I slowly added fish. started with 4 black tetras - waited 3 weeks & added 4 silver mollies -waited 3 weeks & added 4 red wag platys. I had my water tested everytime I added new fish and it was great. Two weeks ago every fish I had died within five days. I now have 6 black phanton tetras - THE QUESTION -how will I know when it safe to add new fish? It is killing me that I have become a fish killer. These are beautiful creatures and I try to do everything right but to no avail. Please any help you can offer would be great.