The vacumes really easy to use and they dont cost much, they just suck all the poo out of the gravel, i quite enjoy vaccuming (i know i'm odd) as i'm alway supprised how much poo a few fish can produce. It also sucks up all the un-eaten food.
I agree with the others, do a bigger water change ASAP, remove any green gunk left from the plant, remove any un-eaten food with gravel vacume (if you have one). Fingers crossed you dont loose anymore, sadly sometimes these things just happen, i lost 3 fish a few weeks ago (2 tetras and a guppy) they had no signs of illness. Catfish dont keep the gravel clean, tbh they cause more waste (mine poo's for england).
I wouldn't recommend changing most of the water, it's usually recommend that you change 10-20% max as changing too much can shock the fish and kill them off. We had problems a few months back and found this link most useful: www.aquariumadvice.com/
It sounds like you have an awful lot of fish despite the 200L tank. Also whatever food they don't eat, scoop back up with a fish net so it doesn't rot and fester in the stones. Usually allow 10 mins for them to eat. We also use a plastic tube to suck up the debris once a week too from in and around the stones and plants where they poo and feed.
sorry,didnt notice you had reposted I was expecting others with more knowledge to have come on by now also realise you may be talking about warm water fish,whereas we have coldwater goldfish. with goldfish,changing water and cleaning out pump if necessary,clearing out plants,giving tank a clean,and putting in appropriate fish medicine should stop further losses,all done as soon as possible. overfeeding isnt good best to get new plants just in case I would have thought
The other 2 barbs have now died - nothing visable that is strange about them. Will do water change tommorow - other fish all seem fine (tetras, minows, catfish, platys, guppys) Had hoped a catfish would help keep gravel clean but think I may have overfed (one large pinch a day - 16 fish) but lots seems to fall straight to bottom. Should I get new plants?
Have a 200ltr tank that has a few mixed tropical fish and some live plants in that get nibbled. Did water tests yesterday and all fine so just put small amount of 'water clear' in and today one barb died and another plus two platys seem ill. Have changed some of the water but worry that the plants dying may have been the cause as stones around them have all gone green quickly but nowehere else in tank. Is it plants, over feeding or the liquid I put in?