i'm going to start at the beggining and work my way through, this may take me a while so i'm sorry if someone else replies before i'm finished and it looks like i ignored your post.
i have read three of your posts now and think i understand your situation. from what i can make out you have a betta in a basin and a 3x1x1ft tank with (at least) 2 goldfish (oranda), a red tailed shark and presumably silver dollars. you set up the original tank, stocked it, did not preform regular maintenance such as water changes and substrate cleaning, then added more fish. the water changes you have done have been a long time between and of 100%. does this sound about right to you?
i also have to ask what you have in the bottom of the tank, is it sand or gravel or nothing? this could be important later on.
i know nothing about your tropical fish i.e. shark, dollars and betta. other people will have to properly advise on this but i'll give it my best.
the first thing you have to do is buy probably 2 more tanks, i say probably as i don't think the betta can be kept with the shark and dollars.
you will need a tank for the betta on it's own
i think 5-10 is the recommended size, so a 2x1x1ft would be more than ample. it would need a filter, heater and possibly an air pump. there are plenty of betta fans on here and if you listen to them they will tell you everything you need to know.
the other tank will be to split your goldfish from your shark and dollars. the goldfish are cold water fish whereas your shark and dollars are tropical. they need different temperatures.
the goldfish i assume are still quite small so the 3x1x1 would do them for a while but they will need a much bigger tank in time. as for the shark and dollars i really do not know what size of tank they will need.
in the goldfish tank you will need a filter and air pump. in the shark/dollar tank you will need filter, heater and possibly an air pump.
this would sort out your stocking.
as for your water problems, a lot of that revolves around the nitrogen cycle, i will not go into that just now as i am aware of a slight language barrier. basically the water in your tank builds up different toxins to your fish, we can not see these toxins, so the water still looks fine and some fish can survive through this. over a longer period of time the toxins keep building and if your fish survive through this they become acostomed to it.
to keep the tank water healthy most people do weekly water changes of around 20%, this removes the built up toxins and keeps the level of them low.
if however you do not do regular water changes and the toxins build up your tank will have what is called
old tank syndrome in this case if you do a large 100% water change you will shock your fish as the water quality is so different. this may be why it looks like water changes are bad for your fish and is also more than likely why your tank smells.
please keep in mind when doing water changes that you make sure the water is dechlorinated and tempature matched.
as for your goldfish. as i said before it could be wen growth, fungus and fin rot and now knowing their tank mates also fin nipping. going on the information i have surmised above i will suggest that it has probably been a case of fin nipping that has become infected to cause fungus and fin rot. goldfish should not (in my opinion) be kept at a tempature above 21C turning the temp up to 28c will not solve any of these problems.
hopefully their will be more people along to help out with the fish that i do not know about