Added Cardinal Tetras Keep Dying - mystery solved.

[Tamaradebra]

I had six healthy cardinals and several other healthy fish. I added new cardinals first from Big Al's (two of three died) then Pet Smart (four total have died - bought three and had a few replaced). I test water regularly and nitrates are a bit high but I do extra water changes. Now my healthy cardinals have early fin rot too.

All other fish I've added have been fine - Corys, Platys, Mollys, Angelfish (one),...

I know Cardinals are less hardy that Neons but this is ridiculous! Any advice? I just love them and prefer them to neons.

[tanksalot360]

Generally, cardinals don't tolerate nitrates well. Using purigen or some absorbing resins may help as a bandaid now and afford you a bit more success. Even better - establish some live plants if you haven't already, they're good to make them feel at home and reduce nitrates naturally. Floating plants are excellent for shading the tank and sucking up nitrates quickly. Also cardinals typically enjoy it a bit warmer 79-82, but too warm you may stress the rest of the community. Btw, your angel may turn on them one day. Additionally, is there driftwood or almond leaves in the system? Or an active substrate? Cardinals typically enjoy a neutral to acidic pH which the above help provide. You can also soften the water and slowly lower the pH by using reverse osmosis or deionizer water with water changes (and top ups - as not to concentrate minerals in the tank). Almond leaves also can help against fungus or fin rot if that's what your experiencing.
Is the tank being filtered by a canister filter? When was the last time you opened it? As canisters can be nitrate factories if heavy with organics.

How long is it before they perish? Is anyone picking on them? You may want to drip acclimate cardinals to rule the transition out.
Once established, cardinals are generally tough tetras, but it's sometime difficult to meet they're needs in a mixed community.

[Tamaradebra]

Thank a lot, LOL. I've added a pic of my tank so you can see I have the live plants, lettuce, driftwood, pH is 7.0, temp is 83, The strange thing is that all my established cardinals are doing very well. It was only when I added the new ones from Big Al's and then Petsmart that I had issues. So something is going wrong at introduction. I watch my tank and fish behaviour closely and the Angel keeps busy picking on one of the Platys, never bugs the tetras.

So besides losing an average 50% death rate of any new cardinals that I buy, on consecutive days, it's a mystery. The resident population is fine.

EXCEPT that they have now contracted fin rot. UGH!

[Tamaradebra]

As Determined following the very sad loss of our beloved bushy nose pleco Spot - seriously - my tank was suffering a slow strangulation from decomposing plant matter caused by not enough light. The new 38g tank I was so proud of came with tragicallyi sufficient lighting to reach the depth and back of the tank. A very sad and hard lesson.