HELP. Water issues in my (previously) cycled tank. Help!helphelp, please.

Without plants, I'm confused as to where that nitrate is going. Have you ever seen NO3?

Water changes and prime will help. Try to keep nitrite as low as possible.

How old is the test kit? Unless you have something funky going on with water chemistry, the only things I can reasonably think of for a well stocked tank that is 6 months old is either user error or bad testing reagents.

Nothing else has changed recently that could have disturbed the beneficial bacteria? How did you originally cycle?

Fancy goldfish have kind of a crunched digestive system, more precisely, they have the same digestive system as a long bodied comet/common goldfish, but shoved into that bulbous body. This can mean digestive issues including slower digestive time. For fancies, it is best not to feed dry foods like sinking pellets or flakes. Instead, presoak the foods. This will aid in digestion and make it so they dont get stopped up. That is likely the cause of them all pooping like mad. They finally are clearing their digestive system out. Fasting them once per week or feeding them foods like shelled peas will also help move things along. I feed my fancies lots of fresh and canned (no sodium) veggies. They particularly like zucchini and french cut green beans. They also like to eat duckweed, so I grow it in another tank then feed it to them a few times a week. I also feed frozen protein like blood worms and brine shrimp in addition to veggies. I rarely feed flakes. Sinking algae wafers are good though as long as they're too big to be eaten right away. They will soak on their own and it will give the goldies an activity. We call it Algae Wafer Soccer Night in my house because they punt those wafers all around the tank trying to bite off tiny pieces.

Goldfish are extremely high waste fish. They are carp, and excrete ammonia through their gills as well as with the poop. Even if your goldies are still small and you have a huge tank, it is always a good idea to have a 30-50% water change every week. Adding live plants can also help keep the system a little more in balance. Anubias do well in my tank. I also have a decent light fixture so I can grow amazon swords. Some of the more delicate plants may become food to goldies though, as they like to eat plant matter.

If you dont already have one, invest in a python or other brand siphon that hooks up to the sink. It will make changing the water in even your big tank easier. Trust me, worth every penny and then some. Also make sure you are dosing prime when you do water changes. Prime is better than any other dechlor as it also removes heavy metals and detoxifies ammonia and nitrites. I usually dose a bit higher than recommended for water changes on my goldfish tank just in case there is any ammonia or nitrite in the tank.

I'd check the lot # on your API liquid reagents...that will give you an idea of whether they're actually expired. The number corresponds to the manufacture date and most of the tests last 3-5 years..although I think the newer kits have exp. dates printed on them directly.