| | Re: New Build Dec 2019
It's hard to answer your question if we don't see a picture of the tank. Many fish from stores are severely stressed when purchased and become diseased.
But still, we owe our fish the best that we can reasonably provide in their new home--your 55 gal. I'm a little concerned by your comment that you started with not too many rooted plants and are now getting volcanic eruptions from the soil. This is the stage--2 weeks after submergence-- where the soil bacteria have multiplied and are going to town on the soil's organic matter.
It could be excessive CO2 in those gas bubbles that is killing/stressing the fish. Or a lack of oxygen for delicate fish. I would start poking the soil to release gas and introduce oxygenated water into the substrate. Make sure that you've got water movement (filter or pump) to keep the water properly aerated. Once the soil settles down in about 6-8 weeks, you can dial back.
Folks, if the substrate is releasing copious gas bubbles, that means the water and substrate could be oxygen-deficient and contain excessive CO2. This is a normal development after an organic soil is freshly submerged. Where it becomes a problem is when you have a deep soil layer or gravel cover, a lack of strong rooted plants, a lack of water circulation, etc.
The onus is on the hobbyist to understand this and take appropriate counter-measures for this temporary, but all-too-common problem. See 'Chaos in Freshly Submerged Terrestrial Soils' section in my book (pp. 130-134).