Sudden die-off, please help

How can you tell your main tank is cycled without any nitrite/nitrate tests?
Since my turtle has been growing fast, this is my third tank in 9 months. Every time I establish a new tank, I treat it with "Stress-Zyme" to jump-start the ammonia-eating bacteria. Every time, ammonia levels consistently test at 0, even though turtles are relatively "dirty." After 2-3 weeks, I start to see moderate algae growth, which I take as an indicator of nitrates. I have no indication other than that. If I implied that I know for certain that the tank is cycled, I apologize. It's just my best guess based on observation and an admittedly limited knowledge of cycling.
 
Find a fish store, and use only items aquarium safe. It sounds as though the tank is not cycled, that takes about a month, try scouping up some gravel from the existing tank that would help to cycle the tank
 
I lost the remaining fry, the juvenile male, and one of the females today. So from a starting batch of about 9 fish (plus subsequent fry), I am down to 1 adult female. I put filter media from my established tank into the guppy tank and I did another 50% water change. I'm trying to test the nitrite/nitrate levels, but I'm having trouble with that. (See http://www.aquariacentral.com/forums/showthread.php?t=179173). Here's hoping the one remaining female pulls through.
 
Feeder guppies rarely survive, so don't be so bummed. The fry however, usually does well, unless there's something wrong. Most feeder guppies I've seen come with diseases. Have you looked for obvious signs of bacterial or parasidic infections? Mine was hit with Columnaris.
 
I stared and stared at these fish while they were dying, and read about all kinds of nasty diseases, but there was no evidence of any of them that I could tell. Perhaps the gills were a bit inflamed, but I feel like they arrived that way and I have nothing to compare to so maybe they weren't.

The last female died last night. When I get home from work I'll be cleaning out the tank. Should I throw away the filter media? I know that it's usually good to have aged media, but in this case I suspect that there may have been something toxic leached into the water from a homemade breeding net. I don't have nitrite or nitrate readings yet because my local store and the next closest one (40 minutes away) were both out of freshwater tests. I may just have to order one, but I doubt I would get meaningful readings by the time I have it so I'll never know for sure what killed the fish.

I'm really torn up about this. I know that feeders often die, but for them to all die so suddenly makes me feel pretty certain that it was something I did (or didn't do), not just typical feeder weakness/disease. I know they were destined to be eaten by my turtle eventually, but I hate it that I caused them to suffer and die for no reason other than my own stupidity. When I saw that the last one died last night, I actually cried myself to sleep. Right now I'm not sure if I want to clean everything up, start over, and try to get it right this time, or just give up on fish and stick to just the turtle since I've managed not to kill her in the 9+ months I've had her. I guess she'd just have to go without live food, but plenty of turtles do.
 
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