You're going to have trouble finding a single food you can just drop in and everyone will have the opportunity to eat. This is not because all of the fish won't eat the same food but because of their feeding habits.
Generally, Loaches and Catfish wish to feed towards the bottom of the aquarium and Betta and Harlequin will wish to feed at the top. This means that unless you have a food that can sit in the middle of the water (or a tablet stuck to the side of the tank) either option you pick won't be ideal for one group of fish.
This is not to say that if you feed flakes the bottom feeders won't get anything but it is a consideration. Bottom feeders will often be much more timid in their feeding habits.
What can you do?
Fish will thrive on a varied diet and the ability to feed them a wide range of foods means that you won't have problems when you run out of a specific type of food because the fish won't be that normalised to one food.
I have a mixture of top feeders, middle feeders and bottom feeders in a tropical community tank and I feed the following:
- Algae wafers
- Sinking Shrimp Pellets
- Tropical sticking tabs (rarely)
- Bloodworm (Very occasionally)
Middle / Top Feeders
- Various flake foods (Alage based / standard trop flake)
- Blanched leaves (lettuce/spinach/cucumber)
- Brine Shrimp
- Diced Scallops (occasionally)
Why are you telling me this? I asked for one food to feed all!
The point is, your fish will grow much better and be much healthier if you provide a variety of different foods. If you really wanted to have minimum foods (which I wouldn't advise) I would at least get two separate foods - one for the regular fish and another for the catfish and loaches.
If the issue is cost (which is understandable as a lot of fish food can be pricey) I would advise you to check out this video which details how to make a homemade food. You'll probably want to puree the end product and think about quantity because your fish will have smaller mouths and stomachs.
I'm sure you've heard this before but it's definitely worth repeating:
It's better to underfeed your fish than overfeed them
This is due to a number of factors:
- Water quality - If you overfeed and there is food lying around in your tank, the food will break down and you could face spikes in ammonia which will actively harm your fish.
- Fish health - Generally a fish will eat all it can because it doesn't know when the next meal is coming. A fish that is overweight is much more susceptible to health complications such as bloat, swim bladder etc.
- Natural environment - If you try and mimic the real world, fish won't regularly find food in the same quantities every day.
A hungry fish is a healthy fish. Most fish will be able to go a week or more without any food. A fish should always be excited to eat food in the tank. If they are apathetic (and there are no underlying health issues) you'll probably want to feed less than you currently are. If they are extremely anxious to eat, you can feed a little more. I will give a tank of about 20 fish (of various sizes) a smallish pinch of flakes and something for the bottom feeders.
I regularly have a day where I feed nothing at all to mimic real world conditions.