So, a year ago, the OP bought fish and they died after 2 weeks, except the cat and snail
Then bought some more fish, and they died, and then bought more that died?
How many times over the year? Just the tiger barbs and oscar, or more?
I wonder if the first fish were infected with something, and died leaving an infective agent in the tank..
But the cat survived (some diseases are species specific), or because it built up, or has an immunity, or may be an asymptomatic carrier.
The snail is probably not relevant, because most fish diseases don't effect mollusks, although it could also be a carrier..
After the 1st batch died, was the tank torn down and sterilized, along with everything in it?
Were any of the subsequent additions quarantined?
I suspect. that....
What ever the something was that killed the first fish, (bacterial?, viral?) is still in the tank.
And every time new fish are added, they become infected and die.
Are there symptoms? or do they just drop dead?
Although the pH is on the high side, I also doubt that it is acutely relevant.
What I would do before adding any new fish, is tear the tank down and sterilize everything the tank water has touched with bleach, including tubes, nets, filters everything and start over. Start a new cycle, sterilize the filter media or buy new.
I would not add the cat of snail (each could be be carriers) , I'd put them in a separate tank.
Add to that....
Since your tap water is high pH, I would only choose fish that are evolved to live in high pH water.
Oscars are not (and get too big for that size tank anyway), tiger barbs, possibly(they can take pH up to 8)
If it were me, once the tank was sterilized and decontaminated, I'd choose some of the smaller rift lake african species because they prefer high pH water, above 8.
I would also do enough frequent water changes to maintain a nitrate level under 10ppm, which would also maintain a stable pH.
Then bought some more fish, and they died, and then bought more that died?
How many times over the year? Just the tiger barbs and oscar, or more?
I wonder if the first fish were infected with something, and died leaving an infective agent in the tank..
But the cat survived (some diseases are species specific), or because it built up, or has an immunity, or may be an asymptomatic carrier.
The snail is probably not relevant, because most fish diseases don't effect mollusks, although it could also be a carrier..
After the 1st batch died, was the tank torn down and sterilized, along with everything in it?
Were any of the subsequent additions quarantined?
I suspect. that....
What ever the something was that killed the first fish, (bacterial?, viral?) is still in the tank.
And every time new fish are added, they become infected and die.
Are there symptoms? or do they just drop dead?
Although the pH is on the high side, I also doubt that it is acutely relevant.
What I would do before adding any new fish, is tear the tank down and sterilize everything the tank water has touched with bleach, including tubes, nets, filters everything and start over. Start a new cycle, sterilize the filter media or buy new.
I would not add the cat of snail (each could be be carriers) , I'd put them in a separate tank.
Add to that....
Since your tap water is high pH, I would only choose fish that are evolved to live in high pH water.
Oscars are not (and get too big for that size tank anyway), tiger barbs, possibly(they can take pH up to 8)
If it were me, once the tank was sterilized and decontaminated, I'd choose some of the smaller rift lake african species because they prefer high pH water, above 8.
I would also do enough frequent water changes to maintain a nitrate level under 10ppm, which would also maintain a stable pH.