Water weed in gallon tank?

i have water weed (maybe hydrilla) in my 1.5 gallon tank... i planted it for design purposes in the pebbles then now its like 4 pcs of foot long weeds... my betta just died and i wonder if it has something to do with the plant... did it take out my fish? did it steal its oxygen? its not decomposing in any way... its even growing... but what effect does it have on my tank and fish.. i chose to keep it coz its so easy to grow, lengthen, etc.. any ideas would be good before i get myself a new betta.. thanks...