FishNoob wrote:
So now we have four danios (never go into a pet shop with three
children and the intention to buy two fish). Two are zebra, two are
leopard. They were put into the tank yesterday afternoon.

ROFL! My BF is like that in fish stores. You'll be fine with four danios.

As of this morning, two of them - one zebra, one leopard - appear
very yellow in colour. I found a photo on-line which showed zebra
danios in both black-and-white and black-and-yellow tones, so
presumably the colouring itself is normal, but is it usual for them
to change colour? They weren't noticeably yellow yesterday. The other
two fish look just the same as yesterday.

Fish like danios subtly change colors all the time. They tend to go
pale when stressed and darken when they're content. Some fish will show
or lose stripes as well. If a bunch of fish in your tank suddenly go
pale and still, they're "telling" you that something is amiss.

Two of them - again, one of each, but not the same pair - seem very
aggressive towards the others, chasing and seeming to nip at them. Is
this normal behaviour?

The non-yellow leopard danio looks fat - would it be noticeable if it
was pregnant? (This is one of the less agressive two, it also seems
to be moving a bit slower than the others, and less interested in
shoaling.)

It's either carrying eggs or unwell. In many small, shoaling species,
male fish are thinner than females and often have slightly deeper color.
If the fat fish's scales start to stick out from its body, it is sick
with a disease called "dropsy." Let's hope that's not the case.

PH is between 5.0 and 6.0 (bit closer to 6), ammonia is 0 and nitrite
is 0.1.

Your water must be very soft. There IS an advantage to having a low pH
while your tank cycles. Ammonia is much less toxic at low pH. However,
bacteria don't grow very well below pH 5.5. If there is not much
ammonia yet, I'd try to gently raise the pH to around 7.

In the short term, since there's no ammonia, add 1/4 US tsp (there's
about 5 ml of volume in a US teaspoon; I don't know how many grams) of
predissolved baking soda (sodium bicarbonate, bicarbonate of soda,
bicarb) to your tank today. The pH and KH will rise and then fall again
over the next 24 hours.

Test pH and ammonia again tomorrow. If pH isn't up to 7 or higher and
ammonia stays below 0.5 ppm (it should since you have some nitrite but
you never know), add another 1/4 tsp of baking soda. This *should* be
enough baking soda, but you can repeat this for two more days if necessary.

In the long term, you have a couple of options. For a planted tank, use
a buffer like Seqchem's Equilibrium when you change water. It's
designed to set the pH at 7.0 and supply essential nutrients for plants.

Another easy, natural way to control pH is to put a form of calcium
carbonate in the tank or filter. It will slowly dissolve and increase
both the general hardness and pH (carbonate hardness, to be more
precise) with no fiddling on your part. Seashells, crushed coral, or
limestone all work. The more you add, the higher the pH will go, up to
a maximum of 8.4. Livebearers in particular appreciate water that's
been hardened with limestone.

Finally, you can keep using baking soda, adding more when you change
water. You can do this if you end up choosing a tank full of soft water
fish and don't need a buffer like Equilibrium.

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com