Well, seeming as ypou've already been given the advice about healing him up, here is my general hatchet care thing:
So, hatchets are schooling fishes, they need to be kept in groups of at least 6. They have a reputation for being difficult to keep alive because of attempts to keep them in smaller numbers. I personally keep them in groups of at least 8 though, since these fish can show some pretty bad consequences when stressed (I've tried keeping them is small numbers and they are plain difficult!). But remember: you can mix the species of hatchetfish and they will all school together!
You see those little 'wings' on the top of their body? Hatchetfish are the only actively flying fish in the world, they use those 'wings' the propel them through the air, whereas Flying fish only jump and glide. A hatchet can 'fly' several meters away from the danger when stressed. Of course, the increased stress caused by small numbers mean
water change time can be tricky. As far as I know, only species from the Carnegiella genus (Marbled, Marthae silver, Pygmy and Dwarf) can jump several meters, although others are good jumpers. For this reason, a tight cover is an absolute MUST!
These fish are easily outcompeted (Platys are peaceful but outcompete them at feeding time!) and I ave lost many hatchets due to malnutrition. They need a carnivorous diet and will not take food from anywhere but the very surface. For this reason, I recommend you feed them carnivorous flake food, and Tetra fresh delcia. Fresh Delcia is really useful when feeding hatchets because you feed it from the very surface. Tiger barbs are bad tankmates because they will outcompete their delicate friends. I find small fishes like Neons and Cherry barbs are good tankmates for hatchets.
I find Marbled hatchets to be less hardy than silvers. I cannot tell you how hardy Marthae silvers tend to be, because I have had mine for only a small amount of time.
A big hazard to your hatchets in a filter! Their bodies are only a few millimeters thick, and they can easily be sucked into your filter to need a grizzly end. I have lost almost an entire school of hatchets to a filter before! So, the answer is to get a special guard for the filter. I find plastic mesh to work well, and cable ties are good to make them tight. Fit this sort of guard to ALL your filters in hatchet containing tanks! They are also useful to prevent small fish being sucked.
There is much debate about the adult size of hatchets. Want to hear my advice? Treat all the common ones as 2". Their bodys are only several millimeters thick, so even if the marbled hatchet does get 8cm (I actually read that in a few places) long it is still a tiny bio load producer.
I find that hatchets LOVE floating plants, Hornwort is an absolutely GREAT plant to have in a hatchet aquarium.
A little word about disease: Marbled hatchets are said to be prone to Ick, but I have not heard of this in other species. This may be another thing arising from the stress of being kept in small numbers, but I am not entirly sure because there is a great lack of info on these amazing fishes!
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