This is my first aquarium, and I'm having some problems. I have a 30G tank, and in that tank I had 9 neon tetras, 6 guppies, 2 twig catfish, and 2 angelfish. The two angelfish were the last to be added. When I added them, one adjusted quite quickly and seems happy (3+ months now). The other one was generally shy, didn't eat, and eventually died after a week. I tested the water, and all my ammon. nitrate/ite levels were negligible, pH = 7.8. Then, a week later, I replaced him with a new angelfish, and also put in a new flame gourami that really caught my eye. Again with the new angelfish, it was very shy, and I couldn't get it to eat. Then it died after a week. The gourami on the other hand was very active, seemed healthy and was eating. Then after a couple of months, the gourami stopped eating and started hiding all the time. After about 1.5 weeks of this new behaviour, it died as well. I am pretty confused as to what is going on, because the water levels are all healthy, I do 20% changes each 1.5 weeks, and all the other fish seem very healthy and happy. The guppies are giving birth like crazy all the time. Any thoughts?