Plants in old fish tank

I had an old empty fish tank doing nothing up the attic so as a bit of an experiment I put an inch of gravel in the bottom and put some young potted houseplants in it such as Fittonia, Crotone, Calathea and Alocasia. I poured some water on the gravel, not too much, and put a lid on the tank, and also placed an old aquarium LED light on top. It's on a timer to come on for about 12hrs per day.

Two months have gone by and each plant is doing very well. When I put a humidity tester in the tank the reading can be anywhere from 90-100%, which isn't surprising really due to the wet gravel and lid, it's literally like a jungle atmosphere in that tank.

However, what I've noticed is that I never need to water these plants, unsurprising really as the soil stays permanently moist due to the humidity. My question is, how do I feed these plants now the growing season as started? We're meant to water when the soil is drying out, but in this tank it never does. I can't water/feed when the soil is already moist, or can I? What's the secret? I suppose terrariums are the same?

 

You should be losing a little water because you don't really want it completely sealed. In some terrariums you can drain the water out and replace it but in your situation you could use a foliar feed or add fertiliser to the gravel layer.

 

That sounds great @Esoxlucius , please share photos!

Plant food in stick format could work, in essence you just push a stick (or half) into the compost and it melts slowly.

 

It's funny because as I was writing my post I realised that fertiliser doesn't just come in liquid form, so I had an idea what the answers would be. Thanks @Thevictorian and @Selleri for your input.

This is the tank at the moment, it's only small. If I have good results over the coming months I do have some bigger tanks that could be used as well. My only issue is when the plants get too big and I have to take them out and put them on a window sill. The humidity difference would be a real shock and I may have problems. We'll see.

 

If they're doing well as they are, I'd hesitate before giving more feed...
Plants mainly eat light!