Norman's Lampeye Killifish; sudden wave of deaths

[Tank-o-Fish]

Hi. Some time ago I started my first aquarium, a 15 gallon tank. After a fishless cycle I put in 10 Norman's Lampeye Killifish 1,5 month ago. Apart from one inexplainable death, the fish did really well the first 3 - 4 weeks. From that point on, each week a fish died. The last week it only got worse:

- One fish was wiggling a bit when swimming. This got worse, until he fired like a torpedo across the tank for a few hours. Found him dead the next morning..

- Another one had obvious symptoms of swim bladder disease, it was swimming tail up and had trouble going deeper beneath the water surface. I fed him only mashed peas for 2 days but unfortunately, it didn't help.

- The biggest one was looking pale for a few days now, not eating that much and swimming very slowly through the tank. Today I found him dying on the substrate, only to swim a bit every now and then. It looked awful so I put him out of his misery. I've added a pic below (he was still alive on that pic). As you can see he looked constipated and his fins were very yellowish.

The other ones were just found death, stuck to the BioFlow filter. I'm pretty sure this filter isn't the cause of death though, since the healthy fish seem to avoid it just fine. I'm down to 4 fish now and not sure what to do. Some of them look a bit pale too. Not quite sure but one of them appears to have a bit of fin rot. I'm pretty sure we're dealing with some kind of infection here. However, I find it strange that every fish had different symptoms. If I didn't know any better, I'd say the water quality is the problem. But apart from high nitrates it seems fine:

Ammonia: 0.25 (always been like this, think it might be the API kit)
Nitrite: 0
Nitrate: 40 - 80 ppm
pH: 7,8

Yes, I know the nitrates are way too high. My tap water tests 40 - 80 too and apparently measures 38 ppm nitrate. JBL BioNitrat Ex is on it's way and I hope it will help keep the nitrates down.

Could the high nitrates be the reason for all these deaths? Any recommendations on what my next steps should be, if any? Thanks in advance.

 

[Tank-o-Fish]

I just realized that I've recently been feeding them some thawed red mosquito larvae every now and small pellets (Sera Vipagran). I bought them after the first deaths since I was worried they didn't get the right feeding, fed only flake food before. Could this have made things worse?

 

[Wolfwood2186]

to me it sounds like your tank wasnt finished cycling. i personally dont like fishless cycling i use cheap hardy fish like danios to cycle my tanks and i have yet to loose a danio to cycling. i am concerned about that 0.25 ammonia and just cause your tap water tests that way to doesn't mean there isn't ammonia in the tap water. id suggest using some seachem prime in your tank it should do the trick as it will remove chlorine and chloramines and convert the ammonia to a safer level

 

[Brookster123]

80 ppm of nitrate is pretty high, I'm thinking your tank needs a little more time to mature, maybe grab a few zebra danios to start and see how it goes??
Edit. Ohh yah and you want to change water to keep nitrates below 20ppm, have you been doing wc?

 

[Tank-o-Fish]

Thank you for your suggestions! The sudden wave of deaths has stopped (although I have had a few more). Not sure what caused it.

However, I got rid of the nitrates. I stuffed some JBL BioNitrat Ex in my filter and amazingly it got the nitrates down to 5ppm! Wonderful stuff, would recommend it.