Thanks for your responses,
I now have had a ghost catfish unexplainably die today. I took a water sample to our local aquarium and they did 2 tests which both came back absolutely fine. I thoroughly examined both fishes and could find no physical reason why they died. Very worried for my other fish!!
I've had the tank since the 3rd October.
It is 200 litres.
I do an 8% water change on a weekly basis, I am running a 5 filtration built in filter and temp stabilises around 26c.
My fish are as follows:
2 Angel fish (medium size)
3 Clown loaches (small size)
1 ghost knife
1 ghost catfish
1 talking catfish
2 male guppys
1 opaline gourami
2 dwarf gourami
2 redeye
2 bristlenose plecos (small)
Any advice to keep them alive will be greatly received.
200 liters is around 52 gallons, yes? It looks like you've got some stocking issues... Here's the main potential problems I see:
3 clown loaches - These guys get much too large for your tank as they get older. They need at least 100g+ for a group when full grown.
1 ghost knife - These guys can get up to 18 inches, are picky eaters, and are very poor at competing for food compared to your other fish. The majority of ghost knives die early in aquariums due to starvation.
1 ghost catfish - You mean ghost glass cat? They're schooling fish that need to be kept in groups of 5+...
2 Redeye tetra - Another schooling fish that need a group of 5+.
2 bristlenose plecos - Plecos are seriously poopers that add a LOT to your bioload.
The current stock may be compatible while they're very young, but you have several there that are going to be stunted and possibly aggressively territorial if kept in there once they get bigger. Also, you have a couple schooling types that will be more prone to disease and nervousness when kept in singles and duos like you have. A stressed fish usually ends up a dead fish.
On top of that, I'd say just bio-load wise you're way overstocked... The parameters may stay ok now while everyone's small, but sooner or later you're going to have the parameters going out of whack faster than you can change the water. Poor water quality inevitably leads to sick/dead fish.
I think you need to figure out which of your fish you want most (of the ones that won't outgrow the tank), shore up the schooling numbers (if you keep them), and rehome the rest.