Why are my fish dying?

[lilsd]

I have a 10gal planted tank that’s been cycled for nearly one year. For several months I had 2 amano shrimp and one lonely Oto in the tank. Struggled a bit with a bright blue green algae (Cyanobacteria I think?) that the shrimp and Oto did not eat. So I doubled down on water changes and trimmed back the affected plants pretty aggressively until it seemed like it was gone. I also removed all floating plants, which there were a lot of at one point, since they were all affected. Last weekend I added a few chili rasboras, CPDs and 2 more otos to the tank. The otos and shrimp are still doing fine, but I’ve been losing a tiny fish every day since then, I’m down to just 2 CPDs and one chili. Water parameters are well within normal range, I’m fairly new to this and I don’t know what the issue is. I have a small HOB filter and a small air stone for added surface movement and oxygenation. I’m not using a heater but the water temp is around 75-76 which should be acceptable for these species. I’ve never had consistent losses like this before. I have noticed a couple spots where the blue green algae is coming back, is it possible that it’s toxic to the smaller fish? I’d appreciate any help as I don’t want to replace or add any more fish until I’m certain that the tank is healthy and not going to kill them, and would like to save the ones that are still left alive. I don’t know where I went wrong here.

 

[GaryE]

I will take an experienced guess. The fish are new arrivals. I do 3 to 12 week quarantines on all my fish, and for pet shop purchases, I expect losses in the first 3 weeks. And that's what you're probably getting - fish that arrived ill even if they looked fine to you and maybe to the pet shop, with an infection working its way through now.

If you bought at a chain/corporate store, death rates will be higher because they count on quick turnover.

I don't know about where you are in terms of leisure, time and pet shop proximity. But I used to go to visit pet shops when they opened, and at at least one other time when I was planning a buy. Let the store do the 2 or 3 weeks quarantine, if the fish aren't selling too fast. Look for diseases, and for dead fish in the tank.

Your tanks can be perfect. But they'll generally tell you they aren't, to avoid any guarantees.

 

[Naughts]

Can you test for nitrite and ammonia? I know it’s an older tank but the removal of plants could leave an uncycled tank.

Can you give us more information about the water change routine? Has it changed since the Cyanobacteria problem was solved? I’ve not heard of Cyanobacteria causing deaths.

Have the new fish shown any signs or symptoms of disease?