It is normal for the gravid spot to change. It is lighter just before the lights go on because the babies move away from the gravid spot (they are attracted to light) so thats why when you switch the lights on, it gets darker, because the babies have moved back to the gravid spot. When this happens, the guppy is usually very close to giving birth. If she looks square from the front or back and is generally very fat, then she is a few days from dropping.
At this stage, I find the best way to save most of the babies is to put a breeder net (an ideal breeder net should be about 7 inches by 5 inches by 5 inches) into the tank, scoop the pregnant guppy into the net and put lots of plants in with her, as hiding places for the babies. Then, check on her every day till you see babies in the net (don't touch the net though! just look into it)
When you see some babies, take out the mother and scoop out the babies into a small net and float it in the breeder net or in the tank (make sure the babies can't get out) and put the mother back in the breeder net and leave her in the breeding net for one more day, just to make sure she has dropped them all. Next day, move the mother guppy back into the main tank and put the babies from the small net into the breeder net again. you can take out the plants if you want. Leave the babies in the breeder net (which is floating in the main tank where the mother gave birth) for at least a week before you move them to a different tank. I have made the mistake with one of my batches of fry of moving tanks the day they were born, and woke up the next day to find all of them dead. Don't make that mistake!
Now feed the babies food formulated for livebearing fry and baby brine shrimp which you can hatch yourself. Feed them 6-8 times a day (roughly every 2 hours during daylight). Make sure you feed them enough, but not too much or it will go to waste. After about a week they should be big enough to eat crushed flakes. Put some normal flake food into a bag and crush it up into fine powder. Usually the babies will gladly take flakes, as long as they are small enough. Light is also important for guppies. The fry should get 14-16 hours of light a day. Also, make sure the temperature is 80F for maximum growth of the fry.
To make a brine shrimp hatchery, just get a normal 2 litre plastic bottle (which probably had coca cola or something in it) now cut the bottom off the bottle and turn it upside down, so the lid is downwards (make sure the lid is screwed on tight!) and the cut bottom is upwards. Now put 2 pints of warm salt water (add 3 teaspoons of normal salt every pint of water) into the bottle and float it in the fish tank (i find that sticking it to the side is helpful) make sure the temperature is 80F in the tank. Also, you need an air supply going in the bottle. Just get a normal air pump for aquariums, some tubing, and put the end of the tube into the bottle (make sure it is near the bottom) and have that switched on 24 hours a day.
Now add 1 teaspoon of brine shrimp eggs and wait for 1-3 days. By then they should have hatched. To harvest them, turn off the air and let the shrimps settle for 10 minutes. Then, get some tubing and siphon out the shrimp. Dinner time, little guppies!
After 4 weeks in the breeder net, the baby guppies will have to be separated into male and female. The male's anal fin will start to look a little pointy, and the females will have a gravid spot. If in doubt, put it in the male group. You might want to get another breeder net for them. Remember, a breeder NET not a plastic breeder TRAP (these are stressful to fish). When the babies start to get a little big for the net, put them into grow-out tanks (5 gallons minimum) they should be about 1.5-2 cm long when you do this. Also, the males should be developing colour by now. Under the right conditions, guppies will take about 6 months to reach maximum size. You can then decide whether to breed them, sell them, or whatever.
Phew! I hope this helps!
