If your tank is that dirty, you're probably putting in too much greenwater. I'd cut back on it. At or about day 10, you should start them on flaked food. Make a coarse powder out of flakes, and give them some along with the bbs. Once they transition to flakes, you can cut down on the bbs by about day 20 or thereabouts. After you stop feeding them live brine, there is no need to use greenwater. It's only to keep the rotifers and bbs alive. Great to hear you've got so many!! Yeah, it's very hard to count them. I usually count the eggs before they hatch. Then, I count the number of eggs that didn't hatch to find the total number of hatched fry. Also, I count bodies to figure out how many died. When they get a little bigger, this method doesn't work if they eat the dead ones. You
MUST make sure they don't learn about cannibalism - it
will be a problem if you don't prevent it early on.
If you have only the one guy, it shouldn't be a problem to put him in with the rest, but give him a really long time to acclimate. Once he's in there, he should fit in with the rest of the crowd. It's never a big deal to mix them if they're only a couple of weeks apart, especially if you only have a small number of the bigger ones. There is no more fighting than usual. If you have a lot of big ones and introduce a few small ones, you're going to end up with a lot of big ones and a small pile of bones

I would add the big guy to the little guys' tank - less fish to catch and less acclimation. Plus, the older guy is better equipped at his age to deal with the change in water chemistry.
When I clean my tanks for the little guys, I use an air line as the siphon tube and scrape the bottom with the air line. I may not get it all, but I pick up all the loose stuff and a good amount of the stuck on gunk. If I need to do a "big" cleanup, I catch the little guys and keep them in a small bucket while I clean house to lessen the trauma. Most of them handle people very well, but there are a few now and then that are very afraid, and you also get the occasional nasty biter. It goes slow, but in a 2-1/2 it's not that bad. I also use the air line to siphon water back in. I think it gives them more time to acclimate to the chemistry change.
Really glad to hear you're doing well with them. I hope you've taken some notes to determine what it was you did differently this time that may have been the reason for your great success. Taking a lot of notes and keeping a log is very useful - it's how you learn the most. Post a picture, I'd love to see them. Let me know how it goes
