Zebra Danio bent fin and Dwarf Gourami strange behavior

Hello all. I have a twenty gallon freshwater tank with a dwarf Gourami, and now 4 zebra danios with a bunch of live plants. My water parameters are:
Ammonia 0 ppm
pH 7.0-7.2
Temp 77-79 F

last time I checked at the pet store because I don’t have nitrite and nitrate test kits but they were
Nitrite 0 ppm
Nitrate 15 ppm

The tank has been cycled for two months now and running relatively smoothly. Today I woke up to a find my 5th danio recently dead so I immediately removed it and made sure the ammonia didn’t spike and used ammonia neutralizer. Now I notice that one of my danios has been hiding in the back of the tank and looks like his back tail is bent slightly up which may be messing with his swimming. Additionally, my dwarf Gourami has been acting strange the past few weeks as he has been hanging out but the heater and most of the time hides around the plants and such. He will also move around the tank in like a jolting motion where he darts forward several times quickly to move around and other times he swims normally. I want to get a bristlenose pleco, a nerite snail, and some neon tetras soon to add to the tank to keep it clean and deal with algae but want to know what’s going on with this stuff. I have been concerned about this and would appreciate any and all help I could receive. Thank you so much!

Invest in a test kit or get LFS to test the water.
I've had huge spikes happen in a few hours more so in less established tanks.
At least then you have ruled out water parameters
Danios are very hardy & ive found they tolerate huge changes in temp & water conditions. So it does sound more like a Infection/Bacteria but best to start with water conditions. Hope this helps & Good Luck

jonny-5 said:

Invest in a test kit or get LFS to test the water.
I've had huge spikes happen in a few hours more so in less established tanks.
At least then you have ruled out water parameters
Danios are very hardy & ive found they tolerate huge changes in temp & water conditions. So it does sound more like a Infection/Bacteria but best to start with water conditions. Hope this helps & Good Luck

Click to expand...

Okay, yes I will probably invest in a test kit soon and test my water when I go to my LFS. I did two 20% water changes this weekend to try to keep the tank very clean. I’ve thought it was an infection of some kind too so I am going to start treating it with melafix to hopefully cure any infections, if it even is an infection. Thanks!