How awesome are you for trying to do the best thing for the fish?! Answer: Super duper awesome!!!
Okay, pardon me. I'm just trying to be helpful... but I'm home from a terrible shift and I am sleep dep-ing. So bare with me.
So tubing... Around here, I often see "airline tubing" sold in a 6 or 8 foot length for $2 or $3 at large chain stores like Walmart or at pet stores. So not to much of an investment there! Like stated, tubing would work well to siphon off waste from the top of fine gravel/coarse sand, but I don't know how it'd be to actually get down in medium to larger gravel. For a small betta tank with medium gravel, I like to use the smallest "gravel vacuum" (a.k.a. "gravel washer", "gravel siphon", etc) I could find. Depending on what you can find, this may cost $3 to $13 dollars. After creating suction, simply shove it in there, twist, lift it up out of the gravel. Repeat. The crud get sucked up, the gravel floats back down. Fish excrement, uneaten food, and other build up will 'poison' your water, so to speak, and need to be removed at least weekly in a combination water change/gravel vac (as others have stated). When you refill, be sure that incoming water is near the same temp as outgoing, and that the water is pre-treated with a "water conditioner". These basically detoxify chlorine, chloramine, and heavy metals.
Another reason that the gravel vac is a good idea- you'll want to avoid washing your gravel in tap water. In your original post it sounded like this is what you're doing. Tap water is treated with Chlorine, sometimes Chloramine. The good bacteria growing in your tank, the bacteria that helps convert Ammonia to NitrIte to NatrAte, lives on surfaces like gravel. The bacteria is killed by chlorine. So every time you wash your gravel in tap water, you're destroying the good stuff. The same goes for your filter pad. And we want to keep the good bacteria! So the best way to care for your filter cartridge is to swish it around in old tank water to remove large particles. And the best way to care for your substrate is to siphon it. Such a small tank with such a large bioload is going to have difficulty cycling (growing enough good bacteria), so we want to help it out any way we can. Be aware that, if you choose to change your substrate, that it might cause a "mim-cycle" because so much goo bacteria is removed at the same time.
Speaking of water quality, I guess I lean the other way from what's been previously said concerning plants. Small tanks (and yours is super small!) are notorious for having wildly flucuating water parameters (Ammonia, NittrIte, NitrAte, etc.) and just generally being difficult to get and keep stable. So, to me, I'd think your tank needs plants! Healthy plants help suck up ammonia and nitrAtes, and produce oxygen. Fast growing plants also soak up nutrients before algae get a chance to use them, so this can help your algae problem, too. I mean, we don't want to totally get rid of your algae before we have your Oto reliably eating prepared foods. Depending on your light (type, watts, K rating, duration, etc), pH, and hardness, we could recommend some plants for you. Pennywort, Anacharis, Water Sprite, Guppy Grass, Duckweed, Frogbit, come to mind but we'll need more info first.
And your Oto... I'm on the other side of the fence on this issue, too. I think they best thing for the fish and the humans is to return him to the store or re-home him. I don't think you can meet his needs with your current setup. Of course, this is just my opinion and suggestion. If you DO choose to send him on his way, let us know and we MAY be able to help you pick out another creature, like a snail or a few shrimp. But this depends, so just let us know and maybe we can figure it out.
If you know someone who has an established and HEALTHY aquarium, and could acquire some substrate and/or some filter media from that aquarium, please let us know and we can help you do what's called "seeding". This is basically taking good bacteria from an establish tank to help jump start your own bacterial growth. Its easy and awesome, to let us know if that could be a reality for you and we'll talk you through it.
Also, it does look like you have a heater. Is this right? If not, we'll have to address that.
So, again, please excuse my faults tonight. Have a fantastic night!