Good question. It used to be that the jags and loisellis would have about 1,000 fry every four to six weeks like clockwork. This was pretty easy to count, given the right method. Count as many as you can, say to 50. Notice how much space those 50 take up. Multiply that by how much space the whole fry cloud takes up.
So then the loisellis started having a lot more fry. Judging the size of the larger fry cloud by the old one gave me arough count of about 4,000 to 5,000. Theirfrycloud was four to five timeslarger than earlier spawns. The newest jag fry cloud is easily twice as large as any of the loisellis efforts. The jag fry cloud takes up about three cubic feet (about 1 foot wide x 1 foot tall x 3 feet wide). It's really insane how large it is. I was truly shocked and amazed.
This is in response for the what do I do with all of them question. I do leave them in the tank. In the past, they eventually would dissappear over three to four weeks. But the last two spawns from the jags and loisellis have been around much longer. Some of the fry, anyway. There are still about 50 loiselli fry around 1" and about 75 jag fry closing in on 2" with a couple even larger than 2". There are four or five loisellis that were hatched back at the end of August '08.
One of my goals with this tank was to be able to get fry to survive to grow out. But, now that they are starting to do just that in relatively large numbers, I am rethinking that. For some reason, I'm not having any luck getting rid of (selling) jag fry locally. The two LFS's take a few, but not anywhere near the number I'd like. I've posted on my local aquarium club site and only gotten one taker. Go figure. I don't really want to go to the trouble of shipping them one or two or ten at a time. It's a lot of hassle for not much money.
I find it surprising that fry are even surviving in there, to me it would seem to be a gauntlet. They must look like bits of food just drifting in the current . They're probably surviving because you booted the shovelnoses.