Don't use chloring tabs, dry chlorine is too hard to work with (rinsing).
You need to completely drain the tank afterwards, use liguid bleach (clorox type), approx. 1 cup per 50g. Clean filters, media, substrate, nets, siphon hoses, little castle and diver, air lines etc. Rinse thoroughly, then rinse two more times. Put whatever is going to be in the tank in it and fill with 6" of water ...If there is chlorine in your water system... (don't dechlorinate yet), let stand for 3 days and check chlorine levels, If high use appropriate amount of dechlor and let stand another 2 days (we are looking to see if chlorine is leaching out of anything), check again, if levels are low, finish filling the tank and check again. If normal rise in chlorine is found treat accordingly and start the cycling process (bio-spira etc.). If you have chloramines treat to remove at the start of the refill then check for leaching.
Good luck and keep use posted,
Dr Joe
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While tank is cycling hopefully the fish are getting better, don't put them back into the big tank untill a month (30 days) after you know they are cured.