I dont have a rescue but i could still give some advise :3
I keep one of my bettas in a plastic container like that which is filtered and heated and is doing fine so i think thats a good start.
For medicine, your most powerful tool will be aquarium salt! As it can get rid of most parasites and such if you know how to use it right.
Also, this is what i do for 2 of my bettas. I didnt have the money for a large tank so i got semi transparent plastic boxes. They are bigger then my plastic aquarium and they were cheaper, less then half the price!
Also, go around checking out different filters and heaters.
My local pet store wanted 3X more money for a filter and heater then at an aquarium store. So look around for the best prices.
I don't know if Crystalicethorn basically suggested this, but on another betta forum I used to belong too, people would always recommend rubbermaid storage containers to new members who couldn't afford a big enough tank for their betta. I'm not sure what the price difference is or if it's even worth it. They could make good hospital tanks if you just leave the bottom bare.
*shrugs* I don't know though. I'm new to all this lol
I don't know if it would even be wise to bother with filters for rescues....8 or 9 times out of 10, daily 100% water changes are required to treat one thing or other....Not only that, since they're set up to temporarily house one betta after another that may or may not have something contagious....everything needs to be sanitized every time...so what's the benefit of the filter that really can't be cycled...other than possibly having the carbon keeping the water clear...? And even that has to be removed while medicating. Same goes for the gravel.... bare bottom would be more sanitary...
I'm by no means an expert...but I have dealt with sick fish...unfortunately for me, I've ONLY dealt with sick fish so far :( and the aftermath of having to clean and/or dispose of everything afterwards....
So I'd say the bare minimum would be best IMHO...nursing sick fish is labor intensive enough as it is without adding any extra work, and not to mention expenses, for yourself...Just some smaller critter keepers or plastic containers, heaters and some silk plants for shelter...and you're good to go :)
Best of luck to you...I do think what you're doing is wonderful...though after losing my little ones...I personally don't have it in me...
I have about a $200 budget when it comes to this, so critter keepers are no issue.
This is really helping me come up with a starting budget.
My best advice - be ready to give them up. I decided to rescue a betta and rehome. He is still in the process of getting better (no anal fin), but it is so hard to think of passing him on to another. I keep saying to myself, "but then I can rescue more". But, I needed to realize I am not the type that can easily foster without becoming overly attached.
I wouldn't bother with filters for them. If I were going to have several rescue tanks, I'd put aquarium gravel on the bottoms of all the tanks just for the sake of having surface area for the nitrifying bacteria to colonize, but I wouldn't bother spending the money for filters. That's just me, though. Some people would prefer having a filter on the tank and no aquarium gravel, but in that case, the filter is doubling as a surface for nitrifying bacteria.
I just never really found it necessary to have a filter on a betta tank, unless you're going to have a large bio-load in the tank because of having several tank-mates (snails, other fish, etc.).
Clear sterilite containers are cheap I was able to get 2 gallon ones with clear lids for about $3.00. I found these at a store called Bigg Lots if you have any. My fish are happier in them than the critter carriers we started with. They are longer and low which allow lot of swimming room. They are at least as long as my 5 gallon tanks. I have taken in some fish that were intermediate in sickness(according to the link I posted below) Without the containers for temporary housing I could not have helped. I like the fish so I doubt I will give them up.
They also have shoe box one with clear lid that are about 1.5 gallons (6 quarts) ones for $2.00 if you need a slightly lower container. Either way the fish has more room to swim back and forth. I cleaned mine with diluted bleach first and then let them soak for an hour with water with prime in it.
The only disadvatage to them is while they are clear with good lighting the critter keeper if you get crystal clear ones allow you to see the fish better. I do have pictures in my album of some of the fish using them. Fiero, Perseus and possibly Curly are shown in them.