oh man, that's my worst nightmare, happened to me once, thank god i was in the room doing some hoovering in the bedroom and saw that the tank was dripping, just a tiny bit. ran downstairs to get a bucket and by the time i got upstairs it was running out about 20 times the speed. had to drastically re-evaluate the bucket needed. thankfully we had a big garden tidy which i kept around as a makeshift tank for emergencies, got everything into that. and even more luckily we had another tank the same dimensions spare in the cellar to transfer everything in to.
but yeah, get the biggest tub you can, get the filter on there and carry on adding your 5ppm for the time being. you may have to kiss goodbye to some of the plants which is always unfortunate but i've found it v hard to keep them alive when they're not in a proper set up.