hey noni...difficult times...
but welcome to the tank...you'll learn heeps here...
i'm a noob...only six months of fish...
my tank is 40 gallons...and i do ten percent water changes...
and in my tank thats around four inches...
now it all depends on tank size and shape...
but i guess that if you want to do a 30% change...
then measure the height of the water in your tank when full...
and then divide that by 3...and there you have it...
so if your water height is say...18 inches...
then you need to swop 6 inches of water...
edit...[remember to dechlorinate and match temperature]...
i do it with buckets and gallons...
it sounds like you need to do a couple of changes soon...
and then read up on cycling...
good luck...be patient...it'll all work out...