Cichlids Are Dying Help! Plz

I am completely new to this (posting a question I mean ). I have had my tank (400 litre) for 5 years, started with tropical fish, but changed over to Cichlids about 2 years ago. Over the last few months I am losing 1 fish after another. Very disheartening. I get my water tested very regularly and it's always perfect. I change water regularly, like every 2 weeks, 25 %, clean filters in aquarium water fortnightly too ( they seem to get very dirty quickly)
I have 5 clown loaches in the tank too, which are nice and bright in colour. My dragon bloods have recently started with black dots, but continue to breed, which makes me think they are healthy plz correct me if I'm wrong . Anyway, I don't clean the gravel every fortnight, not sure if I should be doing that.
I'm thinking of going back to a planted tropical tank with angels etc, but would need some advice on how to restart my aquarium. Any advice is very much appreciated

Cichlids are not my strong point but there are some issues I might be able to help with.
When you say the water is tested regularly I am guessing that you are not doing the testing yourself, but rather having it done at a fish/ pet shop. This can be rather bad as some shops will tell people the water is "all good" but not really understand what the water should be to start with. Please if possible invest in your own liquid test kit (the strips are natoriously inaccurate) then you can be more certain of the actual tanks readings when ever you want to test it. A good test kit will also help you keep track of the alkalinity and acidity of your water.
 
Also water should be changed at least 25% on a weekly basis, and gravel vacumming can be carried out at the same time. Since you have had tanks for a while I am also guessing that you know to only clean the filter media in the outgoing tank water, never in tap water that more than likely has chlorine/ chloramines added which kill the benefical bacteria living in the filter media.
 
From my understanding the cichlids you have are rift valley cichlids which generally  need hard alkaline water. If your water is to soft and acid this could be where the issues are occurring with the deaths.
Clown loaches however require soft water so the mix of the two species is not ideal. Also most clown loaches not only like being in large groups but also can grow to large in most aquariums to properly keep a nice group. Clown loaches also appreciate plants even if they can be very hard on them.
 
I am sure more experienced cichlid keepers will be able to help you further in how to address the water softness/ hardness issues.

 
I change water regularly, like every 2 weeks, 25 %

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I do 50% a week I also have 5 Clown Loaches in a 400 liter tank and about 60 assorted tetras.
 

 
Clown Loaches however require soft water

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+1
They also require a bit more cover and hiding spaces.
 
You have to be careful with Clown Loaches and water conditions, They get Ich easily.

It doesn't sound to me like you are dealing with a water quality issue.   A water quality issue would not likely take your fish one at a time, you would see similar afflictions in multiple fish simultaneously.   Sounds to me like you have some kind of bug in your tank, albeit a parasite or infection of some kind.   You notice any marks on the fish before they die?   White spots?   Lesions or tufts? 

Ech0o, I seem to be thinking the same about it not being a water quality problem. However, my fish do not get any spots (except for the black dots on my dragon bloods, who seem to just keep on living) or anything else I can see, and I do observe the fish a lot. When I see another dead one in the tank I am often very surprised at the particular dead fish, as there was no sign whatsoever, not even 'not eating'

I've decided to divide my fish up in 2 different smaller tanks, with 50% of the water coming from the big tank, and putting one of the two original filters in each tank. I'm rinsing the gravel with fresh rain water, a rinse with salty water. And one final clean water rinse. I read somewhere that salt can kill parasites/viruses. Would this suffice in cycling the water or should I add conditioner to it to help it along?
I have put my dragon bloods in a separate tank as they have spots, but seem to be just keeping on living I want to completely overhaul my big tank and cycle it properly, etc. any hints on that?

NickAU I realise that with the clown loaches, about water quality and ich. Since they are happy and brightly coloured, I'm baffled as to what's wrong with my tank. I will however do my water changes and gravel vacuuming more frequently once I have my tank set up again.

Baccus, really appreciate your advise, yes, will invest in a liquid test kit,
I'm actually inclined to go back to a planted tropical tank, but that's a bridge I will cross when I get this sorted