Hi bigdaddy. That was a lot of fish to put in a tank newly set up. I gather you did not "cycle" it first?
I believe a 29gal US is 36" x 12" x 18".
If it has only been going 8 weeks it should have started off with just the 6 zebra danios and perhaps, at the start of the second month you could have added the 5 other danios. No more fish than that until the filters had built up the good bacteria required.
Please read this link about cycling...
http://www.thetropicaltank.co.uk/cycling.htm
Bala sharks grow up to 14" and your tank is much too small for one, let alone 4. Please take them back to the shop. Juveniles can be raised in a 4ft tank but, to thrive, a 6ft plus is necessary.
Keep checking the ammonia, nitrite and nitrate levels and do water changes whenever they are higher than they should be. Although levels may be good now, as a result of the fish deaths so stocking level has dropped enough for the filter to cope, the fish may have been damaged due to the high readings and just taken a little longer to die than the others.