Thanks for contacting us, and I'm sorry to hear that your little Horsfield has an eye problem. It could well be that a spec of dust or grit has got into his eye, and you could try bathing it with 'artificial tears' -- hypromellose -- you can buy them at any chemist and they look like this:
https://tinyurl.com/y3s99m4q .
Just a couple of questions though, to maybe narrow down the cause:
1. What substrate are you using? If you are using a topsoil and playsand based substrate, which is the one that we recommend, then it can get dry and dusty, so a light spray every day or two can control that. Other substrates (which we don't recommend), like straw, aubiose and other materials can sometimes have sharp edges that can scratch the eye.
2. Where did you put the heat mat in the sleep area? If it was on the roof of the sleeping area that is fine -- but almost certainly not needed at night in most modern homes (the temperature can drop to 12C or 13C at night with no problem). If the mat was underneath the tortoise, however, -- even underneath substrate -- then it should be removed, as tortoises have been known to suffer severe burns from sitting on even low wattage heat mats. They should never have a heat mat beneath them.
If the eye problem doesn't clear up within a few days of administering the artificial tears, then I would think about a visit to a vet -- just in case it's an infection of some sort. If you do see a vet, please ensure that it is a reptile vet (someone who is qualified to treat 'exotics'). If you don't know of one near you, we have a list of recommended reptile vets, organised by county, on our website here:
https://www.thetortoisetable.org.uk/tor ... ajjKmZ7m9s
I hope that helps, and let us know how you get on.
Nina