I have a single SAP puffer in a 20gallon tank. Water Params are as follows -
Ammonia 0
Nitrite 0
Nitrate 10ppm
His water is changed once a week, and he is always very active, and looks to be begging for food. He is the only inhabitant of his tank.
Until 2 days ago he was eating very healthily, and greedily. He's fed on thawed bloodworm, pond snails, and protein rich goldfish pellets (hard enough to grind his teeth a little).
Yesterday I noticed one of his eyes was a little cloudy, and he wasn't making much of an attempt to eat food. I say his eye was cloudy, it just looked a little different. Today the other looks a little odd. I'm wondering now if he is struggling to see his food, because I dropped a pellet in for him today, and as it dropped right next to him, he ate it, and the next one.
Also, possibly significant, possibly not, I have noticed tiny beetle like bugs in the tank. They're small, and hard to see, but I can see them on the tank's filter and on the sandy substrate. They move very slowly (the snails move faster) and they flick occasionally. Any ideas what they are, and whether or not they a significant? The only way they could have entered the tank would have been hitchiking on rocks that have been in the tank since the start.
I am reluctant to medicate the tank, but I'm at wits end. I've had the little guy for about a month now, and he's been a very active and friendly little guy, I'm keen to get him sorted out.

Many thanks in advance. Don't be scared to ask any question, I'm meticulous about my tanks and I'm sure I'll more than likely have an answer for anything you feel is necessary to ask!