Zebra danios are 'shoaling' rather than 'schooling' fish. And as pointed out, this behavior shows up more when they are concerned and stressed. If you keep them in a very large tank, they have a tendancy to shoal more than in a smaller tank.
They are great little fish, and quite hardy, which makes them ideal for fish-in cycling, which it appears you are doing.
While that is not my preference (or the preference of many on this board), if done carefully it can be done successfully with no harm coming to the fish. BUT, it does require a great deal of diligence on the part of the fish keeper in terms of testing, changing water and understanding the nitrogen cycle for it to be done properly.
Quickly: the ammonia produced by the fish is toxic to them, and will need to be converted to nitrite. Nitrite is even MORE toxic than ammonia, and needs to be converted to nitrate. Nitrate is not nearly so dangerous and will be removed by the fishkeeper via normal, routine maintenance water changes.
There are some special features of ammonia and nitrite that you need to know about...
Ammonia as tested by API includes both NH3 (ammonia) and NH4 (ammonium). Ammonia is the REALLY dangerous variety. And the ratio of NH3 to NH4 depends on the pH and temperature. Here's a calculator to help you determine the true danger of the ammonia test reading:
http
/www.hamzasreef.com/Contents/Calculators/FreeAmmonia.php (You will, of course, want to enter "0" for salinity.)
The TRUE goal is to steer clear of 0.02 mg/L, as shown on that calculator.
Then there's nitrite... As mentioned it is more toxic than ammonia. But, how? It is absorbed through the gills and the enters the fish's hemoglobin, which then inhibits the fish's ability to uptake oxygen from the water. This is a condition known as 'brown blood'. This can be combatted with a bit of salt to the water, as the salt also enters the hemoglobin, but blocks the nitrite and allows the fish to uptake oxygen.
What does all this mean?
1 - First part of the cycle, water changes are your best friend (and your fish's). You need to keep a close eye on the ammonia. Use the calculator linked to keep a handle of exactly how toxic the water is. The fish should be quite fine with levels that are measurable on your test kit, assuming that the AMMONIA (NH3) is under 0.02 mg/L. BUT, personally, I'd keep is as low as possible. The bacteria need some ammonium and ammonia available for them to grow their colony. But, you want to keep it as close to that as possible.
2 - Second part of the cycle, water changes will still be helpful, but adding some SALT to the water would be just as effective, and even MORE helpful, as the presence of some nitrite and at reasonable levels (like 1-2ppm), which would normally be toxic will not affect the fish, and will encourage the growth of the bacterial colony. Performing water changes will still be important, just add the removed salt when you add back the water. (So, if your dose of salt was 2 tbsp, and you change out 50% of your water, you'd redose 1 tbsp of salt to replace what was taken out by the water change.)
Can I do anything to shorten the span of this time for the cycle?
Certainly! There are primarily TWO, but even a third.
1 - Get some mature media from a friend with an established tank. The media is the stuff in the filter. The ceramics, the sponges, etc. Its all going to have the bacteria you want. Just place it in your filter, even if you have to remove some of your own media. Make sure you don't restrict the flow of the water. Cut it up however you need to, but fit it in there.
2 - Getting certain products of bottled bacteria have been shown to be quite effective. These products are: Tetra SafeStart and Dr. Tim's One and Only. They have been mentioned by members on this site as effective at SHORTENING the cycling period, and are NOT to be viewed as immediate cures. They CAN be completely useless as well, if they have not been handled properly by the retailer.
3 - PLANTS! Plants can use ammonia directly from the tank, as their nitrogen source. Even if you aren't looking at keeping plants long term, although, I would suggest that even a few hardy plants can be quite useful for the ecology of a fish tank. But what plants? Which plants are useful at using the ammonia? Simply put, the faster the plant grows, the more it needs nitrogen. FLOATING plants, can be some of the fastest growers - as they have the most intense light source, access to plenty of CO2 from the air, and then they draw their nitrogen from the water or air. Hornwort, frogbit, duckweed... these can all be very useful at using excess nitrogen in the water (ammonia, nitrite and nitrate). Short of these, STEM plants are the next best options. But, even the slower growers like java fern and anubias can be useful at this, PLUS they carry bacteria with them and this bacteria includes some of the bacteria you are trying to grow.
Hope this helps. If you get confused, come on back, and we'll help you a bit more.
