I agree that good sized fish were likely added too quickly. That probably caused an ammonia spike Because your tank didn't have enough BB to keep up. Don't be too hard on yourself. I know it's frustrating, but chalk it up to a lesson learned.
And a canister filter is fine, lots of people use them in SW
. Not everyone has room for a sump. You just need to make sure you are very good about cleaning the canister every week. Nitrates under 40 should not harm your fish. For corals and algae control, nitrates closer to zero is best. But anything under 40 won't harm the fish.