So, after thinking about it, here's my opinions, based on your ideas.
I wouldn't drill directly into the bottom of the bottle. I'd drill approximately 2" above the height of your substrate for your filter intake. So if you're going to have 1" of gravel, then I'd drill the hole at 3". At the top, realize that you're going to need to keep the level of the water in the bottle at least 1" below the shoulder of the bottle at a point where the surface area is equal to the bottom area, otherwise you won't have enough surface area to promote gas exchange and oxygenation. So I'd drill the hole for the discharge of the filter so that water returns slightly below that level, to encourage turbulence and promote better gas exchange.
The light down the neck of the bottle may sound like a good idea at first, but I'd discourage against it. If you plug the neck up with a light, you very likely will reduce air flow and negatively affect surface area gas exchange. I think I'd look around and fabricate something that fits on top of the bottle, surrounding the neck to match the outer circumference of the bottle (i.e. to the 10" width of the bottle), and then use "cut to fit" LED light tape. This will actually provide you with more options for light colors and you should be able to get something that will promote plant growth. I still use flourescent lights because I'm old and set in my ways, lol, but the options and customizability of LEDs is amazing. I don't know enough about LEDs to recommend which ones to use.
Now, the heat tape. We used to use this at the refinery, and I've seen it for sale at hardware stores. Are you sure that what you plan on using will be okay for this purpose? I'm saying because we used to have to calculate heat exchange in the pipe we used heat tape on to ensure that heat exchange was adequate to prevent damage to the pipe (for PVC) and that the heat exchange wouldn't overheat (or underheat) the process. In this service, I'd be concerned that there A) wouldn't be enough water flow to effectively facilitate heat transfer and that the tape would get hot enough to melt the plastic, or B) at the low volume of this tank the tape would overheat the water even though the tape is set at 78 degrees. In most cases, when specs say something is rated for a certain parameter the specification presumes certain conditions. So in this case, on a 2.5 gallon set-up the water may actually be raised to 86 degrees because of the volume, but in turn the tape would only raise a 180 gallon tank to 70 degrees. It may also require insulation to ensure the target is hit/maintained as well as to protect from thermal burns. I think you need to take a better look at the tape and what it can actually do for you and how it will affect the bottle and plastic. I'm not saying it can't be done, heck, I think it's innovative. I'm just saying to make sure you've done your homework. Make sure that it's doable and then make sure that you have the right material for the job.
I also wouldn't be surprised if the heat tape would require a 20 amp breaker, so check the circuit you'll be plugging it into even if all else checks out.
As for CO2, I honestly think you'd be better off dosing with a liquid carbon supplement than trying to set-up a CO2 gas injection system.
Is the mouth of the bottle wide enough to get your hand down? I'm just trying to picture how you are going to plant the bottle. Regardless, for general maintenance purposes it might not be a bad idea to cut an access hatch out on the angled section between the bottom of the neck and the shoulder. You could use a couple of glue on plastic hinges and a latch. I also think any additional holes that you're going to drill (e.g. for CO2 injection, additional aeration, etc.) would be better off if they were drilled here rather than the bottom of the bottle, as you wouldn't have to be water tight.
The last six years of my career at the refinery were in process safety. My job was to be the ultimate proponent of Murphy's Law: to assume everything that could go wrong would, dissect the proposed modifications to equipment and processes to determine what could and would go wrong, and then offer some suggestions to ensure that didn't happen. Kinda feel like I'm back on the job, lol.
WYite
