I've influenced a few friends...not the best thing I did, lol! One guy was my friend's brother who just got a 50g tank. My friend was interested as much as his brother. While my friend was willing to listen, his older brother didn't want to listen. He really wanted to mix everything in a tank and asked why he couldn't. I told him saltwater and freshwater fish can't live together, aggressive fish vs. passive fish, predatory fish can't mix with small fish, etc. He wanted all sorts of Cichlids, Goldfish, Clownfish, Lionfish, Tetras...it was a mess. His brother bought some African Cichlids. After a few weeks, I went to visit my friend to find 4 Common Plecos, an ID Shark, 5 African Cichlids, 2 Tiger Barbs, 3 Mollies, and an Arowana...yeah...that tank had a massive die off. The tank is still up, but only has a few fish and they're compatible. When I moved I gave him a few of my fish since my friend actually takes care of the tank now since his brother lost interest. He still has 2 of the 3 fish I gave him 4 years ago.
Another friend loved Goldfish, but couldn't care for them well at all. He had 7 (4 large and 3 small) in a 60g tank. He had 3 filters running, but he would overfeed like crazy and rarely did water changes. I told him he needs to cut back on his feeding. He was worried that his Goldfish wouldn't be able to eat bigger pellets, but he was feeding them tiny pellets and justified the overfeeding because of the size of his pellets. Most of it ended up in the filter. He says he does water changes every week, but the ammonia was so bad! He never cleaned the filters. By the end, he moved and gave up the last 3 survivors to a lfs. I tried to help as much as I could without taking care of it for him.