What am I doing wrong, my fish keep dying?

first it's best not to over medicate your fish, i only use the chlorine remover when i add tap water. clean the water only every 6 weeks and only about 10%. the filter shouldn't matter as long as it keeps the water clean. a fish could go a week without feeding usually fish die from over feeding so maybe you feed it too much. feed once a day and only about 3 small flakes per fish. frozen food is healthier for the fish. algae could be too much food, too much sunlight or the tank light is on too much. for now do not add more fish let the tank sit for 3 weeks before adding more fish if the fish you have now all die then only consider buying one small cheap goldfish and see if he lives. goldfish could be mixed with guppies as long as you don't have too many goldfish so if you have the one that lives he'll be good with other tropical fish. you don't have to put salt in the water so from now on don't add salt just leave what you have. never do a 75%water change that's too much fish need bacteria in the tank. the temperature is a little high i like mine around 75F. i have three tanks and the one tank is 50gallons with all guppies that reproduce naturally in the tank and survive since i have live plants that they hide in until they are old enough to swim with the adults. my fish don't die so it's just to find the right balance and being it's a new tank you need to give the water time to sit without fish or just one goldfish.