I need some help with my little plants, with 2 very different problems, and figure advice from more experienced gardeners will be far better than my patchwork fixes (think I might do more harm then good trying to solve things myself, at this point!).

First, I've been container gardening on my balcony for about 6 years now. It's a west-facing balcony 17 stories up, getting direct (sometimes brutal) sun/heat in the summertime. We've had a slow starting spring so far with lots of wind, so nothing's grown too much yet.

The problem every year is the heat, and dry soil. There seems to be a fine line between watering enough and not watering too much, so I've tried to fix this with a water-retaining soil (Schultz Moisture Plus Potting Mix). In the past my tomatoes seem very stressed by August, as though the roots aren't very happy, and after reading so much about soil on here I'm wondering if there's something I can do to amend my soil and make it better. I plan on trying Al's 5-1-1 next year, but for now I already have my bags of soil and want to use it for the main ingredient, so is there something I can supplement it with?

This year I'm growing some herbs, beans, peas, lettuce, carrots, potatoes, tomatoes, bell peppers, and eggplants. Only the tomatoes, peppers, and eggplants are yet to be transplanted - these are the plants I'm looking to have better soil for. I've already planned on more growing space for each, as past containers might have been too small.

On the flip side I have a planter started back in March with peas, beans, and carrots. I had this in a cold greenhouse for 8 weeks, and think the soil might have become too warm/damp in there, but it's been out for a month now. 6 days ago I noticed tiny mushrooms growing (blech) which I've pulled out as they're found, and sprayed a homemade anti-fungicide on the soil. Now yesterday I found gnats flitting about madly. Never had either of these on my plants before! I know the damage gnats can do to little roots, so sprinkled some DE on, as well as on nearby outside herbs.

Any other suggestions for how to save these from the gnats/mushrooms? I'm letting the planter dry out, but it's been cold and humid out for about a week with more to come. Also, I plan to put my eggplants into the greenhouse in a couple weeks when they outgrow my indoor heating mat, and am worried about them getting too moist and getting fungusy. Any tips there too?

I'll take any advice I can get, I really want my plants to do well this year and am finding myself frustrated at not being able to fix these problems.