I don't know why my goldfish have died
I'm not a beginner when it comes to keeping goldfish. I have a small (550 gal.) pond outside, and a small aquarium in the house.
I'm having a problem that's a real stumper.
About 3 weeks ago I started a small tank (5.5 gal) for 4 small common goldfish, the largest about an inch and a half. It was an unplanned "rescue".
So the tank was running fine. I have an in-tank submersible filer/pump unit, and the tank isn't in the sun. For about 6 hours a day I keep the aquarium hood light on, but the water wasn't ever more than 75 degrees.
A few days ago I noticed two of the fish weren't acting right, they were spending a lot of time resting on the bottom. I checked the pH, it was between 7.0-7.4 (I have trouble judging the color), but it was within normal limits. I checked the nitrate level and salt levels, everything was fine, but I changed about half the water anyway.
Later that night, those two fish were dead. Today, I noticed the two surviving fish were also spending a lot of time resting on the bottom. Just a couple of hours ago one of the surviving fish died, and the last fish isn't looking very good.
I know the standard guidelines for goldfish are 1" of fish per gallon. Only one of the 4 fish was over 1" in length, so I didn't think it was over crowding.
I wasn't over feeding. In fact, I was away for a couple of days over Christmas, and then again for New Years. The water is nice and clear.
Any ideas what's up?