Thanks for taking the time to respond.

Soil is a 50/50 mix of peat moss and standard potting soil, re-potted about 6 weeks ago.

The pot is a "self-watering" pot with a cavity in the bottom, but it seemed to keep the soil too wet so I now make sure to tip excess water out when watering. All the leaves looked quite saggy before doing this, so I don't think it's being overwatered any more.

I've been getting a few brown edges which I think are due to low-humidity, but they don't seem to effect the plant too much.
The main issue is the older leaves underneath seems to develop a lot of small brown spots and eventually die. All of the younger growth seems healthy. I wonder if it could be disease?

There isn't a huge amount of light where it is, so I will try a location with more light and see if that helps.

Cheers!

Do you have another pot you could use? Maybe between the extra peat and self-watering pot, it's just too wet in the roots. As I said, I'm not an expert, but if you repot, I'd skip the peat and use a regular pot with holes in the bottom. Don't use a pot too big. Maybe someone who grows these can chime in; I may be wrong, but it's what I would do.

Your mix is much too heavy. 50/50 potting mix and peat means the soil will hold much too much moisture, and compact too, over time. Use instead half peat and half perlite, or another option is 1/3 small orchid bark, 1/3 peat and 1/3 perlite. A big majority of house plants do very well with this mix for me. Low humidity is certainly a problem. I can see that by looking at the plant. I have the same problem. I can't even grow Calathea because of that. Did you check the roots when you repotted it? Were they healthy?

The roots looked quite healthy when I checked a few weeks ago.

Since I've had so much new growth, I think it must be getting enough light and the soil is more likely the problem. I'll add some perlite and cross my fingers!

I bought the Calathea on a whim without knowing how sensitive to humidity they were. I'm in Adelaide, Australia - so hot dry summers and cool wet winters like LA. I'm hoping it looks a bit healthier as it gets more humid in winter. :)

very late response but you definitely need a humidifier, I keep my calatheas around 60-70% humidity. It needs also Bright indirect light,like a foot or 2 from south facing window is good, also water it when it’s about 2in dry, I do 1in dry during hotter weather. I have 23 calatheas and they are extreme Drama queens ! I get brown edges and yellow tips more than I would like 😂sometimes I wonder why I love these plants when they are so difficult to deal with! So I know the pain but I absolutely love them ,but your calathea looks pretty dry,needs some humidity! Also have you inspected for spider mites ?

Late comment: look like pests is my first thought but the soil not being right will bering its own issues later

Usually with so many replies someone will be well wide of the mark with advice, but all here have offered good direction.

There's little question about the grow medium holding too much water, and perlite isn't going to fix it unless there is so much perlite in the mix there is not enough fine material to fill all the spaces between perlite particles. In fact, as you add perlite, o/a aeration decreases until you reach the threshold ratio of coarse material to fine material (this is when the volume of fine soil particles is exactly enough to fill all air spaces between the coarse particles) which is actually the most difficult medium to grow in. Of the images A-G below, D is the most difficult to grow in (just enough fine material to fill all spaces between the large particles, and G is easiest to grow in as long as the particles are not too large. 1/10 to 3/16" would be about perfect. If you can't see the screenshot clearly, Find it Here.

Click on screen shot to enlarge.

Al