"FishNoob" wrote in message
...
In my tank, there are two sailfin mollies, two black mollies and one
silver molly. There are also three fry of varying sizes, all black,
with the largest having some pale-coloured patches on its abdomen.

I know one of the fry came home from the LFS, but I don't know where
the other two came from. I've been assuming that one of the mollies
in the tank arrived pregnant, but most websites seem to say that a
pregnant molly is very *visibly* pregnant - i.e. it's not something
you'd miss - and I've not noticed any of mine looking pregnant.
Is pregnancy in a molly always unmissable?

I'll do my best here, but I have no great depth of experience with
Mollies. Female mollies, like all livebearers are basically always
pregnant. Even without any males around, they can still have their
monthly deliveries for many months. Whether they really looked very
pregnant would depend on their age, the number of developing fry and
whether they had already started dropping fry (doesn't happen all at
once). You will have to take the web site reading with a grain of salt.

And could fry born at the
same time in the same tank grow at different rates so that they are
now quite different in size?

Completely normal for fry to develop at different rates. Sometimes the
advantage is initially being smaller, sometimes larger, so for the
statistical probability of success, they are born at different times, in
different places and grow at different rates. Also the difference in
growth rate can be attributed to the quality of their early meals.

(It just occurred to me that maybe the largest is a male? - I'll have
to wait for it to come out and see if I can tell.)

One of the black mollies is male, one female. The male has a much
larger dorsal fin than the female. Is this a characteristic of
gender, or is it something he's acquired through breeding?

As far as I remember, it's not a fool-proof characteristic, but very
often accurate. Sailfins are more predictable in this regard.

When he
flares it, his dorsal fin looks almost as impressive as the male
sailfin's dorsal fin.

Behaviour - today I've noticed that the male black molly is being
very aggressive with the female black molly - prodding her to the
extent that he's actually knocked her over sometimes. He's pushing at
her just behind her anal fin. She's taken to hiding down near the
bottom of the back of the tank, which made hubby suggest that maybe
she was giving birth, but I wonder if she's just trying to get some
peace from the male LOL. She looks plumpish, but certainly doesn't
have the "almost square from the front" appearance I've seen
described on websites.

Mollies can be quite agressive with their females, so having larger
planted tanks with many more females than males is the recommendation,
however in a large planted tank and any degree of success, you will be
over run with Mollies who will not respect your male-female ratio. It's
good to keep them with carnivores, such as small African cichlids who
will do their part in controlling the population explosion. Both these
fish like harder water with higher pH.

If we do get more fry (and from today's reading I think it's almost
inevitable, and yes, I can see a larger tank in my future LOL), I
know we should use a breeding trap of some kind. What kind is best? -
the nets which allow the water to flow through? - the solid perspex
kind? - the specific breeding ones that let the fry fall through into
a separate compartment from the mother? And when should I move the
mother-to-be? I know it should be as late in the pregnancy as
possible, but is there any particular kind of behaviour I should look
for?

Breeding traps are more suitable for the smaller livebearers like Guppies
and Platys. I would simply let the fry run around your community tank.
You can adjust the survival rate by adding/removing their primary cover
spots (ie: floating Hornwort, rockpiles, dense vegetation etc).

I am really liking this fish-keeping business by the way :-)

It does kind of get a little addictive ;~).

========

For Jeff (don't delete the OP if you want both easily answered), there
are several species of Mollies (sailfins: Mollienesia velifera &
latipinna, and common: sphenops & latipunctata and lesser seen caucana).
I don't know if they would cross-bred, but I suspect that they would.
Charles might be able to correct or clarify my comments and probably
dated sci-names.

The males will drive females to death in small tanks as they can be quite
agressive. With other livebearers, the Guppy and Swordtail females are
larger, and Platys are less agressive and slower. With Mollies, the male
can be the same size, and a powerful swimmer.
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FishNoob