I am new to Aquarium Advice, new to the aquarium hobby and new to community forums. I recently moved and during the transition I adopted a 55 gallon "established?" aquarium. I have never owned anytime of fish, nor do I think I would have picked this hobby up had it not been thrown at me. I am completely in it now, I have read everything I can think of to google, watched 1 million YouTube videos, I would think with all this "new" knowledge I would feel a tad more confident then I do. To put it frankly, I am nervous and scared for my new fishy family. So I am here hoping to get some guidance and gain more knowledge before I do anything else with the aquarium.
Aquarium story... the tank came with the house
I have research and found that the fish in it are Convict Cichlid (about 17) and one Angelfish (so worried about the Angelfish) and one HUGE sucker fish (don't know the name of him/her yet).
I know for sure the aquarium has not been cleaned, or had anything done to it besides feeding the fish for about 4, maybe 6 months. Honestly, it could be longer, I don't know the signs to look for to be able to tell what condition everything is in. I have had two fish die since they have been mine, about a week and a half.
So far I have only added about 5 gallons of water (per the fish store guys instructions), cleaned the hoods (which to me appear to have never been clean before), (please excuse my terminology, as I learn it will improve) cleaned the tops of two black things that are attached to clear tubes, I am thinking they are connected to the air pump, but I don't feel air being pushed out, so maybe not and the glass all the way around the aquarium. I can not find a filter, I tried contacting the previous owner, but had no luck. I did talk to a neighbor who seems to think the filter is under the gravel?. Everything looks gross to me, the water is more clear from what little bit I have done but the gravel appears to have black yuck stuff all threw it, even in the bottom.
I went out and bought the proper food for the fish, water conditioner, new gravel, I have ordered all knew tubing and decorations. I don't know what to do about a filter, really I don't know what to do period. Since I purchased everything, I have read more. I do not know about water conditions and I am afraid that if I put all new stuff in the aquarium I am going to kill all of the fish.
I have taken some picture, my camera isn't that good, but I can take more and/or a video if that will help. Please someone help me. I don't know what to do, but not doing nothing is killing me.
Thank you in advance!