Bad Mistake

I set up my new 230 litre tank about a week ago. I moved my 7 fish from my previous 65l tank (5 corys and two honey gouramies), moved the filter and also a large rock to the new tank. I also seeded my new Fluval 305 from the old filter.

Things were very steady for the first three days with ammonia and nitrite at 0. I was concerned that I didn't have enough fish to create the necessary ammonia to properly cycle the new tank so went out to Brittas on Saturday. I had the intention of buying 3 fish, but got carried away and came out with 9 (don't tell me, I know I'm an idiot). In hindsight, a >100% increase in the fish population in one go was far from clever.

I got 3 keyhole cichlids and 6 corys. I noticed an ammonia spike on Sunday night and did a 20% water change. All was well on Monday morning, but when I got home on Monday evening, one of the new corys was dead. The other fish all looked fine. Again, I did a 20% water change. Ammonia only spiked to 0.25 ppm and nitrITE was 0.

Tonight when I got home, one of my original corys (my only remaining corydoras napeonsis) was gasping for air at the surface. I moved him to my quarantine tank (which is cycled), but he died a short while ago.

I feel really bad about this. I made a stupid mistake and should have known better. Even worse, it cost me two fish, one of them was my favourite cory. I haven't see corydoras napeonsis since I bought a pair in FishFX over 6 months ago.

All I can do is put this down as a learning experience and move on.

Don`t be too hard on yourself.
We all make mistakes and sometimes we learn the hard way. I know,
I did with with Discus by not quarantining new fish.
Sometimes the effect of ammomnia on fish (epsecially Corydoras) can be irreversible. I seen it with my own eyes when an air line come loose over night in Kinsealy and even the Corydoras I moved to another tank died a day or two later.

Happens to the best of us. Corys are very delicate when it comes to water quality.

I remember when i was starting out i did a water change in an established tank, but i was in a rush and didn't add dechlorinator, wiped out all the Corys.

Don't be too hard on yourself. It's unfortunate but we have all done something similar. We learn from our mistakes, and hopefully through this forum we can pass on our experiences so others can avoid similar issues.

Monty

I have noticed in general in the forum when there is a water quality problem, the first thing that is tried is a 20% water change. When there is ammonia or nitrite where an Aquarius's has done 20% water change, and then final losses the fish, there has been in the end the same mortality rate as the lazy Aquarius's that has not none any water changes.
Lets assume you have 0.25 ammonia or nitrite, and do a 50% water change with water that does not have any NH3 or NO2, you will dilute half of it giving an immediate reading of 0.25/2=0.125ppm (still too high) if you do a 20% water change , you will get 0.25-(0.25X0.2) = 0.25-0.05=0.2ppm still just as toxic and you have probably killed off some bacteria before the sodium thiosulphate in the dechlorinator has full neutralizes the chlorine in the water and you have dilute 20% of the bacteria that was also swimming in the water. If you were lucky enough to have an mature 65L tank, I do not see how you could been able to lets these reading happen.
If you set up the new tank first and waited to the chlorine had vanished and add the ENTIRE filter medium including all the detries, your tank would of by pass in seeding in period, the same applies to a first tank person that gets a bag of live filter medium from the local fish shop.
Corys do not need much oxygen, they can go up to the surface to gulp air like coolie loaches, when kept in wholesalers they are kept in shallow trays without no air stone or filtering in a bare bottom plastic tray in high numbers and 90% of the water is changed every 3 days, the reason they are kept at this stage on their own is they give off a toxin that can effect other fish.

Doing a water is like opening a small window after someone has let for a stick bomb. It makes more sense to leave the room. By forgetting about buying bacteria in a bottle (anaerobic bacteria that require oxygen have as much change as living in a sealed bottle as you have of living with a seal plastic bag over your head), dont beleive in water changes and bacteria in a bottle. Keep feeding down the first 3 weeks in a new tank, and remember my maths that proves water changes are a waste of time, also bear in mind ammonia is not toxic to freshwater fish kept under 7.4ph, but for Africans and marines this is a different mater as non toxic ammonium (NH4) changes into toxic ammonia(NH3), and by measuring the total ammonium and ammonia is difficult to interpret if this was the reason why you loss fish.

Sean,

You're probably right. There is conflicting advice around ammonia in a tank. Obviously it shouldn't be there, but if it is, then some would consider 0.25 ppm over a couple of days to be relatively harmless to fish.

My nap cory was one of a couple. The other died when trapped inside an ornament during the summer. I realise that corys are shoaling catfish and should be in groups of 4 or more. Being the only nap in my tank may have been stressful. He was very much a loaner. Even though there were 4 julii corys, he didn't really socialise with them.

Corys also like mature tanks. Although I added my original filter a some decoration, the gravel was new which is where corys spend a lot of ther time. This again was most likely stressful for him.

In truth, it was probably a combination of all of the above. The new cory may have been just one of those things. Sometimes you can add a group of new fish and one of them dies for no apparent reason. Thankfully all the others look fine tonight.

I would agree with you with regard to water changes. Changing 20% of water when you have 0.25 ppm ammonia is pretty futile. However, when you have a sick fish, you feel you need to do something.

Water changes are probably of more use when there are very high levels of ammonia or nitrITE and 50-75% changes are used to bring the levels down to less toxic levels.

I did another small change tonight as I added Amquel+ to the tank when I found my first dead cory. Again, pretty futile, but I felt I needed to do something. I want to flush it out over the next couple of weeks as it gives false ammonia/nitrITE readings.