It's around 30°C (86°F) outside. They receive 8h of direct morning sunlight, and 8h of indirect light. They are now watered weekly or less with sufficient tap water, and fertilized monthly with diluted normal fertilizer. In colder months (<20°C), they are watered monthly or less.

So I've noticed a slight yellow tone on the skin of the cacti, compared to the green they had when bought. I doubt it's sunburn, since they have not been moved in more than a year. There's also a thinning of the flesh of the ribs.

They have grown and pupped a lot, but they just don't look healthy.

What could be causing this? Is it benign or does it just go worse from here? Could it be the water? Too much? To little? Too alkaline?

I should add that this has happened over the course of the past 6 colder months (never colder than 5°C).

Before and after:

too much sun, not enough water or nutrients

give them more shade and food, they'll green back up

this fungicide below can help too if they start to look like that:
http://www.naturalindust...e&id=13&Itemid=3

Not enough fertilizer can look like that, don't use dilute ferts, these plants like rich fertile conditions and do great with full strength feedings, they can take as much food as any large leafy plant can

it might be a bit alkaline too, if you follow DGs advice they will improve

have you checked the roots for root mealy bugs?

I've yet to see a fungus bother these plants, but perhaps it can