09-13-2011, 12:01 PM
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Check the pH of the tap water. If the tap water is coming out with a pH of 9.0, you will probably need to find an alternative water source, as there are not many fish that will thrive with a pH that high. Luckily a 10 gallon is small enough that you won't have to buy 50 gallons of water every time you do a water change :)
It sounds as though there are a couple things going on here - one of them is definitely the pH of your water. You sound like the 9.0 pH was a surprise to you- is this the first time you've tested it, or was it a spike? What was it beforehand?
If your nitrates are reading 0, it likely means that your tank is not cycled. You may want to take the opportunity while you have fewer fish to do a fish-in cycle to get the bacteria in there. There are plenty or articles on this forum and people here willing to help and answer questions. It will be better for the fish and will stabilize your water conditions.
Lastly, I hate to say it but you have some stocking issues. You don't say how long you've had the fish or how long the tank has been set up, but I have a feeling it's relatively new. Danios and cories are both shoaling fish. Danios should be kept in groups of at LEAST 5, and cories in groups of at LEAST 3. In a 10 gallon tank, that's about your stocking limit right there. You don't say what kind of danio, but in general danios are not well suited to 10 gallon tanks, they are either too active or too large.
If you can provide some more information about your tank, the equipment in it, maintenance schedules, etc. in addition to testing the pH of the tap water I'm sure we'll be able to help you get things under control.