I have goldfish in a tank. I added cichlids 3 weeks after. They died over the weekend. I don't know if it was the heater I have in there or I could have overfed. My question is what do I do to next time keep my cichlids alive the next time and should i do something to the tank before I put new ones in? By the way goldfish are still living.
Goldfish are in the carp family and are coldwater fish, they thrive in temperatures from 15 - 25 °C.
Goldfish handle water temperatures above 25 °C poorly, so they are not a good fit for aquariums at all. For goldfish to really thrive, a pond is the best.
Goldfish are seen as dirty fish as they produce more waste than many other types of fish, so they need a larger amount of water than other fish.
Cichlids are tropical fish and they thrive at temperatures from 24 - 30 °C so they are ideal aquarium fish. The difference in optimal temperature for cichlids versus goldfish makes the two types of fish incompatible so you can not expect these two types of fish to live in the same tank.
Please do not mix tropical fish with cold water fish, it is not possible to fulfil the needs of these two species in the same tank.
I would have no way to know if your heater was the problem as you asked because you never told us the temperature of the tank. My tank is at 24 - 27 °C (75 - 80 °F) it fluctuates with the temperature swings in my house, but it also has a heater. I have 2 goldfish in there along with my new cichlid, 10 minnows (hopefully will become cichlid food when cichlid gets larger, I bought too many and can't catch them), 4 zebra danios, 1 gold danio, 2 giant danios, some ghost shrimp which I think the cichlid ate and 1 map turtle as well.
All these fish (besides the newest only cichlid) have survived with the turtle for a few months now. The cichlid seems to be doing fine. My plan is to make my turtle tank more of a cichlid and turtle tank without all these smaller fish. My turtle caught some of the minnows but now rarely chases them because he thinks he can't get them and would rather beg for food.
Besides all those minnows I don't have that many other fish, only the 5 danios and the 2 giant danios. I plan on relocating the goldfish as they get larger into a pond my neighbor has with other goldfish and koi in.
My advice is to keep your water very clean. I have to do so many water changes and also have 2 filters running in there, but still do biweekly water changes, and every other day or so I do waste vacuuming.
I have a large parrot cichlid fish *6" living with 3 huge goldfish and 2 koi fish. They are all healthy and have been living together for 3 years now with absolutely no problems and no aggression. So , I would suggest you look into what your water temperature was. The goldfish are hardy. They can deal with the warmer temperature. The cichlids need extra filtration to be used, double what your tank requires because the goldfish produce a lot of ammonia. Was your tank cycled?