Tetras

[Rebeccabug]

I have had an established planted freshwater 29 gallon aquarium for appoximately 3 years. I have had two cories and three neon tetras for approximately two years. I got three more neons and an albino pleco about three months ago. I gave away the five platies we had about a month ago. Two weeks ago I bought five glowlight tetras. One of the glowlights died three days after purchase, which I didn't think was too abnormal. We were away for three days; when we returned yesterday one of the older neons (one that we had had for two years) was dead against the filter. I immediately did a water check and ammonia is registering 0, all other levels (nitrates, nitrites, ph, etc) are within the safe range, with water at 76 degrees. A second (one of the 2+ year olds) died last night, and I checked the water again, again water levels were safe, and did a 25% water change. All of the remaining tetras (both breeds) appeared to be shoaling well together and all appeared to eat well. This morning one of the newer neons (purchased three months ago) was dead. It does not appear that any of the fish have any signs of ich or other visible diseases, no signs of breathing/gill issues. The cories and pleco are active and the remaining tetras are still shoaling. I'm concerned about losing three neons in the last three days. Does anyone have any recommendations?