I've seen many use sand, even though they don't need it. It depends if you're getting one from the wild opposed to who raised it in a tank. Really fine substrate like caribsea eco-complete planted substrate might be an alternative. Myself i would do large gravel stone, because earth eaters like to sift through the bottom of the tank at times. Question where was you going to place the earth eater?
Here's a quote from the book South American Earthers, in the Satanoperca daemon section( I presume by 3 spot eartheater, you mean S. daemon.
"S. daemon is a black water species which prefers slow-flowing, tea colored, calm waters with sandy, muddy or loamy bottoms".
Salsberg found them in "fast flowing water.
Preferred water parameters are soft water, with a pH, between 4.5-6.5
So an aquarium with a sandy bottom would be preferred as opposed to gravel.
This species sifts sand for small edible animals, not large ones, and even though the mouth seems large, it prefers small edibles.
Another quote from S American Eartheaters (I paraphrase).
"this species requires a very fine/soft substrate, they should be kept in a group, and when choosing companion fishes, avoid excessively large species, as S daemon is not very well able to defend itself and may waste away in such company".
Take these quotes for what they're worth, but it seems to me, you are again choosing a species with water requirements similar to the Luciocephalus you just lost, and trying to put them in with a large much more aggressive species, over a substrate that will also cause stress.
Thus again, setting yourself up for failure.(sorry to be so blunt)
There are Geophagines that would better suit your water, and your intended oscar tank mate (if your tank is large enough), and substrate (unless you change to fine sand).
I would suggest those from the Geophagus braziliensus group.
For either species if you choose to poo poo the books suggestions , my hope is the tank is at least 6 ft long. Below Geophagus iporaguensus one I would think is tough enough to be housed with an Oscar
We are all limited by our tap water to some degree.
There are species that are very adaptable, and these are the bread and butter species most readily available.
There are also those exotic types that because they are finicky about exact conditions, are often hard to come by, or you don't hear much about.
Some like oscars do well for a while in less than ideal conditions, but can become chronically scarred with HLLE over time if not cared for properly, you have only to look in the disease section to see questions like, my oscar was perfect for 2 years, but now its fiull of holes, and listless. Why?
Or why now is the aggression killing my stock?
Often you can get away with 1 out of the norm less than ideal condition, if you make up for it with excess water changing, or only one or a few individuals in large tanks.
But once you get into more than one stressor, like opposite water parameters than ideal, combined with inappropriate tank mates, lack of tannins for black water fish, and maybe a lack of enough water changes conjuring up high nitrate, this is pushing the envelop.
The best thing to do is after you have done enough research, choose fish that fit your tap water, provide the proper size tank (or the right size fish for the tank you have) , and provide pristine water conditions for what you end up with.
If you do those things, you won't be posting in the disease section, or asking why things have all gone to hell.