if she dont know much about the tank, just switch the 20g long into the 29g... she wouldnt know the difference... just tell her the tank grew because your fish grew =P
you should ALWAYS put your family first before anything, i do agree with your mom about you being late for dinner, dont be late to family stuff, if you have enough time to look after your fish before dinner, than there will be time after dinner... put it on hold and just get back to it
you have to know what comes first... and if you dont put family/school first, than i dont know if you really know your responsibilities yet
i agree with lisaz, you dont even have a income and you have all those tanks... who is paying for it? your parents of course, put in the fact that you have a couple filters + heater and w.e electrical things running 24/7... think of the bill, and you adding another tank... that increases it... so i do agree, get rid of 1 or 2 tanks and just add your 29g... dont be so greedy and want NEED NEED NEED without giving