Growing Areca in a terrarium questions

[KikiTiki]

Hi all! Totally new to palms so forgive my ignorance. I germinated these Areca seeds (Dypsis lutescens according to the seller) which was very fun and they are now seedlings. I'm growing them for the purpose of putting them in a terrarium. I know, probably not a great idea as they should outgrow and require constant maintenance, this is mostly an experiment. 

I currently have them in small pots and am finding their growth to have plateaued quite a bit. They're planted in a 50/50 peat/perlite mix (probably not the best mix but no one has died). I'm wondering if it would be better to plant them in the substrate in the terrarium which is a similar mix. The only issue is there is a lot more substrate and it probably stays on the moist side despite draining okay. Maybe I try adding something to the terrarium substrate to reduce water retention? The amount of moisture that goes into it sort of needs to stay consistent for the sake of the animal I keep in there.

I'm looking for any and all input. Their roots are coming out the bottom of the nursery pots and look very healthy. They get pretty strong LED light (maybe not enough?) and the terrarium stays around 80f/27c. I have slow release fertilizer pellets in each pot. Any suggestions on how to accelerate their growth or is it patience I'm lacking? Thanks all!

Here is a picture of them early on, kind of cool to see them grow like that. germinating them was very fun

[happypalms]

Your soil

mux sounds pretty good forbb be indoor growing. 
Your choice of palms to grow inside , they get rather large and difficult to hold back in regards to growing out of control. Perhaps a Dypsis minuta or a small growing palm to begin with would be a better choice. A little chamaedorea such as tenella a much better choice and you should be able to find one somewhere.