if your starting over i am assuming that you are taking back the damsels, or already did, but you do want to take them back. you can get cured or uncured lr
, but if you buy it online, you will get some die off, causing ammonia/nitrite spikes. with no fish in the tank, go ahead and put the rock directly in the tank. theres no reason to use a trash can to cure it if there are no inhabits in the tank. it will also alolow the critters in the rock to move into the sand to escape the high levels in the water.
you dont need to get rid of the canister filter. youll actually want it to use to move the water around. it doesnt sound like you're doing a reef setup since you got a puffer, so you dont need to worry about nitrates as much as you would if you had corals. if you do want corals, i would remove the media inside, but still use it to move the water. you do probably want to get a skimmer though.
will provide for biological filtration not mechanical. inverts will provide mechanical, but since a puffer will eat them, they wont help much.
basically you just want to be patient with the cycle. test the water regularly and wait until all amonia and nitrite levels are zero. do some water changes during the cycle so levels dont get too high and kill off your critters. when it levels off and reaches zero. do another water change to get rid of some of the nitrates.
when you add fish you want to make sure to do it slowly. you normally want to add only one fish at a time, unless adding paired fish, or two more agressive fish. and add them at intervals of at least two weeks. this will allow the bacterial to catch up with the bio load. adding too many fish at once will cause spikes and be harmful to the fish.
and dont worry, itll get better for you.