Depending on your light, 12” is potentially too close, especially for seedlings. You mention the ac infinity 3x3 tent. If you’re using their S33 light that fits that tent, 12” is too close. They actually recommend 18” at 40% for seedlings (see below), though I have found that to be too close for some plants and generally operate at @20”, which can still be pushing things in terms of ppfd. Your light may be too close for the seedlings. They’ll take it better when they’re a bit larger and you slowly dial things up. And lose the dome like everyone recommends. I’ve never used one at all.
Heat mat can definitely help with the seedling stage - I used one around the clock for the first two weeks or so on my current grow (I’m still pretty new to this too). Removed it once the roots were really spreading out, as I can not adjust heat in the mat and the soil was actually getting too warm, which could promote unhealthy stuff in the root zone as I understand it (and one of the plants in particular was not enjoying the experience.)
In my current grow, all AC Infinity stuff, 3x3 tent like op, S33 LED, I’ve had the light generally 18-20” from the top of the canopy. I’ve had to turn it down or raise the lights a few times when the plants were showing signs of too much.
Posting a recommended height/power level chart from AC Infinity. This is for the 4x4, but should be similar. Just suggestions, as individual plants will react differently.
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