We have fairly new 20 gallon tank (set up the end of December). We woke up yesterday to 3 dead glow fish. Now, the other fish are all acting lethargic and are either at the top of the tank or on the bottom. Our one snail is floating at the top but still seems to be alive and the other is just below the surface not really moving. The water temp is about 79 degrees & the water tests all come out pretty normal. We did have an algae problem & the nitrite was high so we put in new gravel, changed the filter, & did a 25% water change & it seems to be clearing up. We’ve also been replacing fake plants with real ones. This is my first time having a fish tank so I’m not sure what to do.