i have had a 20 gallon tank set-up for a while now and my friend is giving me a 10 gallon that has been sitting around. can i transfer the 2 platys, 4 neons, 2 emeralds to the 10 gallon or would that be overstocking? i want to make my 20 gallon tank into a semi-aggressive tank. what if i kept one of the cory cats in the 20 and only had 1 in the 10? would that be good?

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"just do it"

You should be fine moving them all over. But, if you want to be extra careful, you could always leave one of the corys in your 20g and move it over later once you are sure that the others are doing fine.

thanks, BTW do you think the 10 gallon wont be over crowded?

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"just do it"

Neons are very small so they don't count very much toward your stocking level. Following the general 1 inch per gallon rule you should be fine as long as your emerald corys aren't very large (I'm not that familiar with that species.). Approximately how large are they?

yes IF you either use the filter from the 20g or the gravel for a while(gravel forever or whenever). Use 10g of water from the 20g and just do a PWC on the 20.
No not overstocked, also the corydora wont get more then 2" max if its a female in those sized tanks.