African Cichlids, need to be in a high
pH water, i.e around 8 ish, low Ph's are not good for these fish. Also Regular water changes are needed. Gobies will be ok as they are ok with neutral and lower PH values..
Also set up a rock face using seperate rocks at the back of the tank to immitate thier natural habitat as much as possible. It's my personal view, but.. I wouldn't advise mixing African cichlids with other species.. esp because of the habitat differences, i.e. ph,
GH, substrate
Can you find the sex of the two cichlids that are ok? It may be shaking in one spot as a part of its mating ceremony or to warn another cichlid off.. clues are golden looking egg spots on the anal fin for males( Malawies), other species the male may be more brighter in colour than the female, eg. lake victoria species..
Most importantly though, 20%
water change, wait 1 hour for conditioner to disperse then add hagen/nutrafin cycle.. Also you could make sure your carbon in your filter is in good condition, renew if not. I don't know what other people think here, but I personally dont like using carbon in the filter as it can mask other problems :/ water changes for the win every time
Good luck