Ok, I just scored 7127 and passed this level. I did it by concentrating on speed more than chaining so much. You still need to try to make chains to pass it but I was letting too many people line up at once to see what they wanted and trying to make too many chains. I concentrated more this time on speed, not selling any extras that weren't asked for, and also I sold the rainbow birds to anyone that had a question mark whether it made a chain or not. I also sold the rainbow fish when a customer wanted a fish of any color. Keep trying, YOU CAN DO IT!!!!!!!!!! This will be a tough one to get expert goal on but sure it can be done with practice.

eek, I can't even get close to goal, much less expert. yikes.
I've tried all the hints, but still no go. I may give up.

I've also tried all the tips and am still short of making goal. This is so frustrating grrrrrrrrrrrr

I finally passed all the levels but still need to get Expert on many of them. On level 51 I only gave extras at the beginning of the game when there weren't customers lined up outside and when they asked for them. As someone else said, I gave out rainbow birds to all who had a question mark and rainbow fish for fish with question marks. I would also go ahead and pick up 2 dogs when the "big" ladies came in since they usually ask for those. In level 52 I keps 1 dog on the vet table so it would be ready when someone wanted a +dog.
Tried to chain as much as possible and always clicked on the "pet" house as soon as that came available.

I finally got goal on level 51, barely. It took me about 25 tries, but I finally made it and finished the game. I'm going to take a break before I go back and try to get expert on the levels I missing.

Anyone have any tips on how to get expert on this level? I'n only lacking this one and 36, 37 - the impossible ones!!

I haven't quite made expert, but I've come close with the following tactic:

1. serve everyone as fast as possible - forget about upgrades except for the first two people.
2. chain cats, then birds, then fish in that order. You should be able to chain all 5 cats, 7 or 8 birds, and 4 or 5 fish.
3. always give a rainbow bird to ? people. You need to start with at least 15 rainbow birds to do this.

Here is how I passed and got expert:

I got rid of all fish, rabbits and hamsters. This not only helped me pass the level, but I blew the expert score away. I actually was able to serve so many customers that I ran out of colorful birds.

Thanks for the advice, I too have achieved expert on level 51. I still had rainbow and yellow fish though. It seems stupid that a successful store should have so many people leaving angry because the pet they wanted isn't there though. Hamsters aren't very good pets anyway I guess as they are nocturnal, and lots of people don't realize how much work rabbits are, so I guess that could justify it? Mind you hedgehogs need a lot of attention... Oh well, thanks again.

Hears the trick thats guaranteed to work buy 20 rainbow birds and about 5 of the other birds then empty your inventory of all the other pets then play you will be chaining rainbow birds for the whole level, and customers will find their requests not available so they will leave quickly leaving you only with birds:) I was able to pass the expert goal with this trick