I set up a 20-gallon aquarium in May this year. I started off with 3 tiger barbs, 2 hi-fin red-eyed swordtails, 2 albino corydoras, 2 red-cross tetras and 2 golden rosy barbs.

Within a week, the 2 albino corydoras (suspected poisoning? due to inflamed, red gills) and 1 red-cross tetra died (it’s stomach was cut open so I assumed it was attacked by another fish).

A week later I added 2 albino sharks and 2 red-cross tetras. They all did fine for over a month.

Then one morning I woke up and one of the golden rosy barbs was bleeding (one the left side by the fins). I isolated the fish for awhile in another tank and treated the wound with medication. When I put the fish back in the tank, the other fish attacked it until it sank to the bottom of the tank.

A few days later, I woke up and one of the hi-fin red-eyed swordtails has died; it had already turned white and was hidden under an aquarium plant. A few days later, I noticed that one of the red-cross tetras has its scales coming off and its tail and fins nipped. I isolated that fish in another tank and treated it with medication thinking it had some fungus/fin rot and it died two days later. Just then my last golden rosy barb had its tail nipped (by the tiger barbs?); it survived though, and the tail has just grown back.

Thursday afternoon last week I bought 2 more additional tiger barbs after reading from a website that their minimum number in a tank should be at least five in order not to be violent to other fish. I also added 2 scissortail rasboras.

However, the following day, one of the scissortail rasboras died, finding it at the bottom of the tank. Then on Sunday morning, I find one of my albino sharks dead. Its belly was bluish-blackish and when I took it out of the tank, it smelled really really bad.

Today I get home and find my last red-eyed swordtail dead, but it didn’t turn white like the other one did.

I have already cleaned out the tank, the filter and changed the water and added water conditioner in it. I feed the fish twice a day.

I’ve been to many fish shops and consulted the people about this matter and one said that it’s the tiger barbs who’s killing the other fish. Also, I only get my fish from two fish stores and both always tell me that my fish are compatible with each other (or maybe they just say that just to earn money from me).

I don’t know what’s wrong. Either my fish aren’t really compatible with each other or I introduced fish too soon.

This is only my second aquarium. My first aquarium was only a 5-gallon one and I took care of 3 neon tetras without any problems for almost two years (until my 6-year-old niece decided to dip her hand into it and killed two of them). Later on I had 3 red platys and they got along fine with the remaining neon tetra (the person I gave them away to said one of the platys got pregnant twice already).

What am I doing wrong? I’m about to give up!!!! I’m ready to get rid of the tiger barbs and go back to caring for neon tetras and platys.