I've been running a 30gal tank for about 9 months (new to the hobby) without any issues short of build-up of algae/cyano requiring cleaning. After treating for algae (Tetra Algae Control -- 3 doses over 9 days), on 3/22 (post the last treatment), I added two new algae eaters to my tank -- one L091 red-fin pleco and one clown pleco bringing my tank total to 12 -- 2 angels, 6 dwarf neon rainbows, 3 clown plecos, and the L091 red-fin. 4 days later, two of the rainbows were dead on the bottom of the tank. My water levels tested at: ammonia=0mg/L, nitrite<0.3mg/L, nitrate=25mg/L, Phosphate=1-2mg/L, pH=~7, temp ~80degF.
While doing the tests on the water, one other rainbow acquired a yellow/orangeish color cast
(VERY noticable), turned upside down, and fell to the bottom of the tank -- gills twitching. Moments later, another rainbow started having breathing difficulty. I performed a 60% emergency water change (treated with 2x the usual new water dose of Seachem Prime Conditioner). The two fish perked up, but died hours later in the same manner. When a fifth neon started exhibiting the yellow/orange color change and breathing difficulty, I did another 50% emergency change. I took the fish exhibiting symptoms to the local fish store and they noted raised scales and white lips on the rainbow but did not have an idea what would cause these particular symptoms and such rapid death. Within 24 hours, all 6 rainbows had died (the first 4 within 6 hours of each other).
The LFS suggested treating the tank with an alternating combination of naladixic acid/K-mycin and an anti-parasitic (formalin/malachite green). After acquiring the anti-biotics and before even starting them, the L091 was on the bottom of the tank with breathing difficulty (5 days post-purchase). Water levels now test at: ammonia=0mg/L, nitrite<0.3mg/L, nitrate=12.5mg/L, phosphate=0.25mg/L, pH=a touch under 7.0, O2=~8mg/L, GH=3dH, KH=2-3dH, temp=~75-76degF. I treated the tank as suggested yesterday with naladixic and formalin/malachite. This morning my L091 was upside down on the bottom of the tank dead.
Any ideas as to what would cause this? I'm suspicious of the two new fish, but the LFS tells me there's no way that anything they carried would sweep the tank in 4 days killing that many fish. At the rate that my fish are dropping, I'm afraid I'm going to lose them all... Thoughts?