I just wanted to agree with everyone else. But as long as they are babies
 the 20 should be fine for now but I'd definitely get in the habit of doing a 
regular water change. To many its a shock how much water change you 
should be doing, as most pet stores will only tell you like 25% a month but
 that isnt really a good idea. Weekly is best if you can pull it off. Many do 
50% weekly, 25% is probably fine for the average tank. You might be 
good with biweekly also but I'd not recommend any less than every two 
weeks at 25% minimum. Some with overstocked tanks actually do 2-3 
water changes a week, and ~50% each time! I dont have the time to pull 
of more than 1 wc per week but I can usually keep up with once a week 
and I'm very lazy. 
I often say the single best thing you can do for your fish is a regular 
weekly water change. Just make sure you match the temps close enough 
and use a good water conditioner like Seachem Prime.
BTW, have you heard of a python? Many newer aquarists dont do 
waterchanges since they are a PITA. But they also dont know about the 
Python, which attaches to a nearby sink and will drain and fill your tank via
 a hose and you dont have to carry buckets or anything like that. 
And also, I hope you have plans for a good sized tank! If you planned on 
keeping all those same fish together in one tank you going to need a 6 
footer probably (125-135g minimum). 
Good luck! And dont hesitate to ask anything on the forums if you have 
more Q's.  
