If you post some pictures of the fish it might offer more information.
Fish losing weight over a period of time is usually intestinal worms and or gill flukes. Section 3 of the following link can take you through the steps of treating for intestinal worms. Treat all your tanks at the same time because if one fish has worms, they all have worms. Deworming the fish and treating them for gill flukes (worming medication does that too) will remove one possible cause of deaths.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
Fish dying a short time after new fish are added to that tank usually die from an external bacterial or protozoan infection. New fish should always be quarantined for at least 2 (preferably 4) weeks before being added to an established tank to stop the introduction of diseases into the established tank.
It's preferable not to mix several species of dwarf cichlid in the same tank because they are territorial and quite often fight. Even if they don't rip each other apart like bigger cichlids do, they still stress each other out and that weakens their immune system and makes them more vulnerable to diseases and death.
If you have lots of fish in a tank, even babies, you need to do lots of big water changes and gravel clean the substrate. Fish like suckermouth catfish (bristlenose) produce huge amounts of waste that can encourage protozoan parasites to explode in population, and lack of big water changes and gravel cleaning can let the pathogens get out of control and start killing fish. Your fish might not have an external protozoan infection but if you are keeping lots of fish in a ank, you need to do big (75%) water changes and gravel clean the substrate at least several times a week. The filter should also be cleaned at least once a month. this helps to minimise the number of disease organisms in the water and reduces the chance of the fish getting sick and dying.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
BASIC FIRST AID FOR FISH.
The following should be done if you ever lose a fish or the fish get sick.
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.