I'm kind of new to all this. I have a 20 gallon tank, 5 platy, 3 zebra danios, a frog and a snail. It's been setup for about 9 months now. I don't know the actual numbers - I use the test strips that show everything is ok...sorry, I'm still new to this!
Ok according to the strips I use: the first nitrate reading is 20 which says safe, the second nitrate reading says .5, hardness shows 75, alkalinity shows 120 and pH shows 7.2
Okay, let me just add Zebra Danios need a 4 foot tank at least, because they are highly active and prefer lots of swimming space. Also they should have groups of 6+ ideally.
Just thought I might say that
I have an idea, but not sure how well it would work, if you put your syphon in the gravel to suck out the dead fry (Probably should do that ASAP as dead fish will leave ammonia, which is toxic) into a bucket, and from there use a flashlight or something bright and see if there are any live fry in there, and from there possibly net out the fry into something like a small tank or a breeding net. Note, if a small tank might want to include a cycled filter and a heater.
Test strips are mostly inaccurate. When you say "First nitrate reading" and "second nitrate reading" do you mean NitrIte? I would assume the second reading is nitrite. And a cycled tank should have 0 nitrite. A nitrite reading would mean you also have ammonia, which is toxic for the fish. I would advise you to do a waterchange asap, and also invest in a liquid test kit, especially since it sound's like your tanks in a cycle.
Hope some of this helps.