Sounds like ammonia or nitrites to me. The water changes are URGENT, and so are the test numbers.
On a second note, when I first started buying these fish, everyone told me they get big but no one ever said how big. I had a 55g when I bought my first tiger SN and was told that would be fine. This is a fish that reaches 50" and likes room to move. They are known as a tank buster in that they will knock the end out of your tank when they get crowded. They are a beautiful fish and don't grow as fast as a lot of fish but even your 180 will be drastically too small at some point. I'm not trying to burst your bubble, I love these fish and have owned a couple in past times. I just wanted to let you know what to expect. There are several other types of Shovelnose that don't go as big and are just as much fun. I gave up on the tigers several years ago because they are just too big for a home aquarium. also they like live food and tall plants to hide in. I fed mine cichlid pellets and he did OK but when the feeders hit the water he always got real serious about hunting.