Tropical Cichlids

Question
Dear Lynda,
I'm new to this hobby with fresh water fish tank 55g with cichlids.
I started the tank cycling with an oscar and aulounocara. Things were going well, ina a couple of days the water got cloudy and then suddenly it just got clean overnight and we left it like this for one week and then I introduced another four fish (blue acara, red fin shark,zebra %26 aulonocara# and everything was fine for a couple of days then they started getting white spots but over a period of four days the spots were getting better almost gone and then suddenly they satrted getting worse and I started treatment with protozin and increased the tank temperature to 30dgrees C and removed the charcoal from the filter, I lost the oscar the next day, have completed the teartment 1,2,3 %26 6 day yesterday. Checked the water chemistry and it seems good PH 7.5, omonia 0, nitrite 0, KH 15.
Could I have done something different? I have a couple of questions
When should I return the charcoal to the filter.
When I do water changes 20%, every two weeks, I put aquatan in the tap water, should I leave this water for two days and the introduce it to the tank or I can directly put it in the tank
How often should I clean the filter and should I clean all the media #sponge, charcoal and ceramics) together. How often should I change the sponge, charcoal and ceramics. Is it necessary to change the ceramics.
Also need to know how much aquarium salt to put in and how often.
When should I introduce new fish and in what quantity, the tank is now 4 weeks old.
I like angel fish, would they get along with the cichlids.
Thank you for your time and awaiting your reply.

Answer
Hi Ajas,
First, I must tell you that an Oscar needs a 60 gallon tank all to himself in order to live a happy life, so he would have been in an overcrowded tank if he had lived.  When we do not give the right amount of space an Oscar needs, even when small, his growth is stunted, his internal organs have no place to grow, and he dies a horrible death.  
When cycling a tank, never cycle with fish, this is cruel.  I know you didn't know this, but the best way to cycle a tank is to buy a used sponge that fits your filter.  Squeeze out the access water, do not rinse, and put it directly into your filter, and let the water run, adding water that as evaporated for about 4-6 weeks, until you have zero ammonia, zero nitrites, and nitrates are low.  When cleaning your filter, never rinse your sponge under tap water.  Rinse it in water that you have removed from the tank.  This way you keep the good bacteria in your sponge.  A sponge is good for a very long time, until it is old, ragged, and falling apart.  You should change your charcoal every month. The ceramics, you rinse, they stay good for a very very long time.   
You will know when your filter is dirty, usually depending on the fish you have, it should be cleaned every month, sometimes more often, you will see.  25% water changes every week are a must, always siphoning from the bottom.  If you have a good water conditioner, you can add the water right away from the tap.  I use Aquasafe, or stress coat...there are other good water conditioners on the market.  Make sure the water is the same temperature as the water in your tank when adding the water.
You cannot mix African Cichlids with South American Cichlids.  You will have to read up on the aulounocara.  The red tail shark is very aggressive as he gets older, should not be mixed with any fish in my book.  He should have his own tank.  An Angel fish would be ripped to pieces, no Angel would survive with an African Cichlid, and a Red tail Shark.  An Angel lives in acidic water, this means the PH should be between 6.5-7.0.PH  African Cichlids live in Alkaline water 7.5-8.5PH  
Before putting fish into your tank, you will have to read up on the fish you want to keep.  Not all fish get along, not all fish need the same water conditions, or decor, or temperature.  Some fish are fin nippers, some are violent, aggressive and can kill, and some are very territorial.  These are very important factors when adding fish to a tank.  You must know the size they will grow to when adult, and you must never overstock your tank.  Overstocking brings stress, and stress fish get sick.  Eventually they die, so be very careful not to overstock.  Like I mentioned to you above..and Oscar needs 60 gallons of water all to himself, 2 Oscars you must double this to 120 gallons, and so on..
When adding fish to a tank, you must do this slowly.  Depending on their size, you add two fish, wait one week, check your water chemistry, if all is well, do a 25% water change, add 2 more fish, wait one week, check your water chemistry, do 25% water change, and so on.
Look at the fish you would like to have, make a list, and I could tell you if you can mix them, how many you can put in, and the fish they are compatible with.  
African Cichlids are not a beginners fish.  Your african cichlid is aggressive, but less aggressive than most African Cichlids.  If you plan on keeping him, then you could go with the African Butterfly (Anomalochromis thomasi) and the African Acei, also the Lemon cichlid  (Neolamprologus leleupi) Keep in mind that you would need 3 females to a male.  This is very important when keeping African Cichlids.  If you do not do this, the male will harass the female until she is so weak, she will die.  These fish are mostly mouth brooders, and you must have a tank ready for the female.  She keeps the fry in her mouth for about 21- 26 days.  It's lots of observation, lots of study.  
The red tail shark, I would either keep him in another tank, or take him back for a credit.
I hope this helps.
Lynda