I have two dwarf puffers, one male (poison fish), one female (tasty fish) in a relatively small tank (~25L). The tanks has plants and some decoration for hiding places and to break up the line of sight between the fish. These puffers have been getting along great for the last 8 months and only seem to fight every now and then. (I have not noticed any bites/injuries)

Last night as I was heading to bed, I remembered that I had not replaced the lid on the tank, to help with evaporation. As I turned the tank light back on to see what I was doing, the male fish was startled and seemed to disappear. I noticed that he jumped and landed on the bracket for the air pump. I was quick to tip him back in and he was only out of the water for maybe 5 seconds. I put this down to startling him with the light. I will tape off the landing on the bracket so he can not beach himself again.

This morning, the male fish has been swimming around erratically and darting at its own reflection in the tank wall.
Every now and then he was swimming fast to the surface and would jump.
He was not attacking or bothering the female, who seems her normal self.
Usually the male is quite shy and just hovers around the tank.

So now I am concerned that he has been like this since the night before.
he has calmed down now.

Is there a possibility that the male is unwell?
There is no abnormal markings/colourations.
He is not gasping for air or anything like that.

He could also have poor vision, he will often hunt for the reflection of the food.
Could he be attacking his own reflection thinking there is another male in the tank?

My tank parameters are below.
NH3/NH4+:0
NO2-:<0.3mg/L
PH:8
Water Temp:25.5degC

Care
Food: Defrosted Blood worms once per day.
Water: 30-40% Changes weekly (whilst vacuum the rock bed).
Filter rinse/changes: When required, visual inspection.

Hopefully I am over reacting.
Thanks for your advice and help.

Regards
David

The lid should be on the tank 24/7, not just to help with evaporation, as you've discovered.

You say that he has calmed down now. My guess is that he was freaked out for a bit and he's fine now.

I always forget the chemical formulas for ammonia, nitrite, and nitrate. Which is which? If that's ammonia that is .3, then you have a different problem. Ammonia can burn the eyes, which could be the reason for the vision problems; nitrite can also cause vision issues and erratic behavior. Also, if it is ammonia, then you seem to have an uncycled tank. How long has it been running?

LilGreenPuffer,
Thanks for the reply.
I do normally keep the lid on 24/7.

My bad about the chemical names.
NH3/NH4+=Ammonia
NO2-=Nitrite
NO3-=Nitrate (I have just realised I have not been testing for this! And so could have a Nitrate problem)

The tank has been running for 8 months.
I may not have cycled it correctly initially as I thought I was testing correctly.
When I replace a filter I leave the other filter behind it for a week to help bacteria settle into the new filter. I have a product called Zirca water which is some bacteria supplement that was recommended at my local pet store.

When I replace the water. I use a de-chlorine chemical and allow the water to sit for a while, usually over night.
Apart from the Zicra water, all other products are Tetra.
The reason for this is, I live in Japan and can find English info about the products on the web.

Looks like I will be heading down to the pet store for a Nitrate test kit.
Will update on the results.

Regards
David

Ok, I have checked my Nitrates, 10mg/L .
I did a 50% water change as part of weekly clean.

The male ate food tonight but the female was not hungry.
The female is now swimming around similar to what i described earlier.
The male is as well. I am quite concerned.

Closer inspection of the female has protruding bump under the chin.
I searched on the forums and found a similar more extreme photo where Nick suggested it was a tumour or Goiter. I added some salts as explained, but weaker than directions. I did not want to stress them too much.

Body shape of both fish looks healthy apart from the chin.

I will keep a close eye on them.

If anyone has any other ideas, please let me know.

Thanks
David

Make sure to keep ammonia and nitrites at zero, even if it means doing a 80-90% water change twice a day. Nitrites can cause erratic behavior, as it often affects the brain. Puffers are highly sensitive to ammonia and nitrites and can die quickly from exposure. Nitrates are fine under 20ppm (or 20mg/l, whichever abbreviation you like - they're the same thing).

Zicra will not help anything. The bacteria doesn't need to be added at every water change - very very little bacteria is lost through water changes. Also, nearly all bacteria products are absolute bunk. Those that are not are useless a day after you open them.

If you think it's goiter, go ahead and treat at full strength.

If it's goiter, you'll want to get the salt water reef tank iodine supplement as soon as you can. My fish with the goiter lived a normal lifespan with no further goiter growth after treating him with the supplement. I used it once a week after water changes, about half as much as recommended for a reef tank.