Hi all, I am new to the aquarium world. I made the common mistake many new hobbyists make and bought a 10 gallon tank and overpopulated it. (20 fish). They were fine for about 2 weeks when I did a 25 percent water change. That same day all of my fish were acting sluggish and laying in bottom of tank. Long story short all but 2 of my fish died. They got white fungus looking stuff on them and died. After doing some reading I realized it was probably high ammonia. So I did a 100 percent water change. (did not clean tank walls gravel or plants). Then I took my water to pets mart and had them test the new water. Lady said everything looked great. I bought 3 new tetras, and 3 zebra danios. I made sure to put bacteria starter in the water, and I put conditioner it it also. I acclimated the new fish to the new temp before placing in tank. The next morning all 3 tetras were dead, and the remaining fish were all sick. The rest will probably die in the next day or so. There isnt any more fungus stuff on the fish, but I am unsure why they would all die so quickly. I did a new water test this morning. Ammonia was about .5 Nitrate was 20 nitrite was 0, hardness was 150, chlorine was 0, alkaline was 180, PH was 8.4. All my fish are dying. What do I need to do to stop this cycle? Should I empty tank and clean it thoroughly? Very frustrated. I know I probably need a bigger tank, but 10 gallons is all my wife will let me have right now. Thanks for your help.