Does this SCOBY look okay?

I've been growing thia scoby for a few months, I split it into 2 about a month ago. I checked on them today, and my larger one has some odd looking pattern on the top. I think it looks like a biofilm, but I just want to make sure that the biofilm isn't being caused by some other bacteria trying to take over my scoby. Before I noticed it, I took a sip and it tasted okay, just like it needs more time to ferment. Thoughts?

A scoby IS a biofilm. Additionally, microbes don't "take over" a scoby, they "take over" the source of food (i.e. sweet tea in this case). Your kombucha will be full of varying bacteria and yeast that all aim to "own" the food source and outcompete the other microbes; that's how they survive. Depending on the microbe in question, the food source may be depleted quickly or not so quickly (some microbes can act on food sources that others cannot). Additionally, as the acidity in the solution grows the harder it becomes for the microbes to keep living.

All of this is to say, as long as you're not seeing mold/fuzzy/furry/fluffy stuff growing and your fermenter is open to the environment (but covered with breathable material) then you should be safe to drink. You will want to replenish the food source more frequently than "months" though to keep a nice balance of microbes at work in there.

Awesome, thank you. I was thinking other bacteria might be able to form a second biofilm over the original one and "steal" the yeast. Happy to hear that is not the case.

I have been replenishing the food source, pouring out 60% or so of the tea and refilling it with fresh tea. I just meant the scoby was started a couple months ago. It's now almost 2cm thick, so I've decided it's time to make drinkable tea, rather than the super strong, vinegarey stuff I was letting it culture in before, so I poured out around 90% this time and refilled it with tea that was slightly less sweet than what I was using before. (I started it in 450ml extra strong tea with 10g sugar, I'm now using 450ml regular strength tea with 5g sugar.) I just replaced the rubber banded cloth cover with an airtight cap because I think it tastes close to done, and I want it to be carbonated. I was planning on leaving it like that for maybe 72 hours, and releiving the excess pressure every 24 hours of so, to avoid an explosion.

Thoughts?

Oh, I understand now. I thought you had made up a batch of tea a few months ago and have just been letting it sit since then.

As for the ratio of water to sugar to tea: It is certainly flexible but you want to ensure that the microbes can create a hostile enough environment that other (harmful) microbes cannot easily take over. Part of this means giving enough initial food supply that they can create that environment. What exactly that is? I'm not absolutely sure, but I've seen it suggested that a minimum of 1/2 cup (~100g) per gallon of water (~3785ml) is recommended. That would suggest that you should use ~12g (minimum) of sugar per 450ml water/tea, just to ensure enough food supply for good microbial growth and to ensure the microbes "own" the tea. At least, that's how I currently understand it.

Carbonation still has be baffled. Sometimes they carbonate well; sometimes it's just a little; and often time (lately) they don't really carbonate at all - and that's WITH adding additional food supply at bottling time! When they did carbonate, I was generally finding 5-7 days about right on the counter before putting into the fridge.

So, I gave it a taste today, and it was pretty good! The other one I had that hadn't been sealed airtight yet was actually slightly carbonated too! I ended up taking half from each bottle to fill up a whole bottle to drink, after pouring them back and fourth a bit to try and get the leftover kombucha somewhat homogeneous, they ended up with about 1/3rd in each due to my multiple taste tests. Topped them off with fresh sweet tea, increased the sugar to 12g per your suggestion, (let it become lukewarm before adding it) and rubber banded paper towels over the top to start the next batch. I'm pretty happy with the results of my 450ml cultures. I might up-size at some point, but for now I'm satisfied with what I have. I think they are growing extra fast as a result of the small size and the nearly 80F cupboard I have them in. The resulting product was (unsurprisingly) brownish yellow, looked similar to beer, but with strands of culture. Actually had a taste that reminded me a little of beer, it obviously tasted more like tea, but with some interesting "caramely" undertones. Not sure how else to describe it. I might try adding some ginger and raspberry juice to one of them after the next batch to see if it ends up tasting more like the store bought stuff.

Sounds like a success! It definitely takes some adjustments here and there to fit your process/schedule, but the end results can be splendid when done right. It's like sourdough bread for me, as the kitchen temps change throughout the year then so does the process and timings you need to use. I guess that's the nature of living things