help.. 12v batter will not charge..

[yotasan]

i used my fish finder battery to run a bilge pump to drain a 300 gallon aquarium. i figured it would have enough juice in it but i was wrong.

any ways, i hooked up the bilge pump and lets say we set it and forgot it.. came back to a half filled tank and the pump stopped working.. uh oh, battery is dead.

tried to charge it but the charger beeps low voltage and will not charge. any recommendations? the battery is 12v 10.5 amp.