loss of fish

Question
I have a 29 gallon tank with 3 cichlids, 2 plecostomus and 1 ancestres.  I have had it for 3 years without ever a problem.  Ever.  My neighbour looked after it for a week in Mar (she is very knowledgeable of fish).  She said that one fish showed signs of ick and treated it.  1 week after I got home, some of the fish looked like their scales were scraped.  2 days ago 1 fish died, yesterday 3 more and today the rest of them.  Yesterday am I did a half water change and the filter intake valve had not been in all of the way (filter still seemed to be working).  Pet store owner thinks maybe lack of oxygen.  Any ideas?  Ph seems to be where it always is.

Answer
Hi Sheryl.
I am really sorry for the loss.
This is really hard to to analyze without other water numbers.
Did you check ammonia?
You say the filter was still working, but do you mean it was just running, or that it looked like water was coming out of the output?
If the water was coming out from where it was supposed to be coming from, then I do not think it was oxygen loss.
What really comes to mind here with these fish dying so rapidly is some sort of poisoning.
What did your neighbor use to treat the ich?
The scales looking scraped doesn't sound like any disease,it just sounds like the fish were scraping at the gravel to ease their discomfort.
This is a tough one, but I don't think it was oxygen loss. They would have responded to the water change. I think it was some sort of poisoning like nitrites or ammonia or medicine use. Let me know if you have any more info.