Unwell Puffer Fish

Can anyone give me some advice, I have a breeding pair of emerald puffers (tetraodon cutcutia) over the last couple of days the female has spent 90% of her time lying on the bottom, not eating and has turned very dark. The male is perfectly happy and trying to breed with her. I also have a pair of red eyed puffers and a single South American puffer. The emeralds have always been top of the pecking order and very bold. I'm assuming she might be stressed but not really sure what's causing her to be so stressed whilst all the others are fine. Any advice would be helpful. Thanks in advance!

Also, what are your water parameters?

The tank is 160 litres, quite a few plants, large rocks, bogwood and shells, she is able to hide in the shells, under the logs and rocks and will rest on the leaves on some plants. The male spends most of his time resting on the leaves while she swims but she is currently just sat barely moving. I have seen her swim completely normally but then goes back to the bottom again and stops moving. She's normally the paler of the two, as with all emerald puffers, he changes colour throughout each day but she is almost black all over now... They also seem to over produce mucus when they're stressed which she has started doing but not too excessively.

Another possibility is that she may be constipated - I've had fancy goldfish who were prone to this and sat on the bottom for days, when it turned out that all they needed was something to clear out an intestinal blockage which was pressing on the swimbladder ie making sure that they had more veg and live food (eg chopped peas, spirulina and bloodworm) and less dried (flake/pellet) food than usual. [Having said that, if ill, they can also be just as you describe.]

Is there any possibility that she could be full of eggs? I know that some species of fish will tolerate a "salt dip/bath" ie half an hour in a shallow basin of water with the appropriate dosage of salt to relieve the situation. [I wouldn't do this just yet, though, until you've checked whether this might be a possibility and that puffers are able to tolerate this.]

As Littlefish said, knowing what your water test results are will be important too - it might just be a matter of ammonia or nitrite or nitrate having crept up and she happens to be the first fish to be affected, so it'll be really important to know what your results are to help with any diagnosis.

I have to start work shortly but Littlefish and others will hopefully help further.

Thanks all, she appears to have regained some colour and is hiding away in a corner under a rock. It's my sons second birthday this weekend so getting a testing kit or treatment might be difficult but I will try and get to the shop and see what I can do. My local has a puffer expert who has helped me with breeding them before so hopefully has some ideas. Appreciate the advise and will keep you all posted!