5 Gallon Tank Platy Woes - Beginner

Hello everyone!
Super newbie here... in fact I'm the mom of a super duper newbie. I appreciate the info I've gleaned the last couple of months from you all and am now dipping my toe into this world for help.
Background: Classic story, my son excitedly wins a goldfish at a school event earlier this year (RIP Blaze). He came home with Blaze in a cup so Gma instantly shows up with "supplies." A horrifying bag with a plastic fish bowl, a baggie of glass "pebbles," and a container of Top Fin Pro goldfish crumbles. Blaze was doing well...ish. He was fast and behaving normally. We made a deal that if he could care for Blaze for 3months (fully expecting poor Blaze to pass on at any minute), he could save up for a decent tank. He got a small plastic (I know) plant and to our surprise three months later, Blaze was still alive and we were looking forward to his upgrade.
Gma #2 was visiting and decided to take him to the pet store to pick out the new tank even though I required research first.
Short story even shorter, Gma was a little excited and was easily sold. They ended up with a 5gallon starter tank (filter, heater, etc), three more plastic plants, actual rock pebbles, Top Fin Quickstart, and "friends" for Blaze. My son called me excitedly at work to tell me they were washing the tank (with dish soap - oh no!) and it would be all set up that evening.
Guys, I tried to make it stop. Even with my minimal knowledge.
Sure enough, Blaze and friends were all together in the new tank, sans filter because no one noticed the "min fill" line. I tried to get the little guy to research to create a harmonious tank but... well Gma was visiting. Filter working, our goldfish, platy, Guppy, and a snail.
Blaze did not last 24hrs. There were several days of loud mourning and a service for him. My son blamed himself. It was all very sad.
A few days later, the guppy got pinned by the cascade. It was so very very upsetting.
The platy (Don) and the snail (Slo) are still alive.
A trip with a friend to the fair brought home a prize goldfish and before I knew about it, s/he, Don, and Slo were roomies.
The first hour went well, everyone getting along, then I saw it; the nipping and pursuing began. Don (platy) was honed in on the goldfish. But the time I got him out, the goldfish was missing too many scales. He did not make it the night. My son stayed up with him out of worry and hope.

CURRENT: 6 weeks later
Don is now focused on the previously ignored snail. As of this morning, Slo has barely moved and gets nipped when trying to emerge. As far as I can see, there isn't any damage and he is a very large snail.
Is there any way we can temper Don's agression? He is the last remaining fish and I don't want to just pass him on to the next owner. With all the destruction, I personally don't want to keep him but my son is committed to caring for his fish and snail. We also need time to save for each additional purchase... but I have read that a 3:1 female:male ratio is best. Also don't want to flood the tank with fry and death.
Help please.
You'll notice the large decoration in the middle has been tipped onto its side... Don did that. Unbelievably he did it. He just kept ramming it until it fell over.

 

welcome to the forum! I have four male platys without any females. it seems that each one has it's own personality! it seems pretty much like a hit or miss afair. I guess you just got an aggressive platy. not much you can do. although the aggression could also stem for too many fish for the tank. each goldfish needs at least 5 gallons, each platy one and a half, and one gallon for a guppy. the aggression might be stopped by an upgrade to a bigger tank, but again, no promises.

you could try more platys...

 

At this point the platy has the run of the tank now that's the rest have passed on. 5 gallons for just him and a snail.

 

So, I'm a total newbie to this, but, if it helps, I think your platy is half platy and half swordtail from the picture. The aggression issues might be because of that.

 

almost all platies and swordtails are crosses between the two. although you never know... especially with platies.

 

So, I'm a total newbie to this, but, if it helps, I think your platy is half platy and half swordtail from the picture. The aggression issues might be because of that.

When checking it out I read that Don may be a Platy Wagtail, which it think has to do with his fin coloring rather than parentage.
If he is a hybrid, which seems very likely, what would you say are the main characteristics sparking that thought. Is it just the agression or something else?
He does look quite slim and the photos I've seen of platys are usually plump around the belly. Also, the space between dorsal fin and mouth seems to be a pretty sharp line. I'm also noticing for the first time the tailfin has a bit of a pin shape that is not super prominent... I think that last point is more of a platy characteristic but just an observation.
Note: I keep saying 'him' but really haven't got the slightest idea if he is male or female.

 

So, when I looked at the picture of it, the tail had a slight point, and it looked slimmer than most platies. The aggression also made me think it might be half platy half swordtail. I think it's a female (but it depends on how old it is)

 

Well I don’t have anything I can add to help but I do sympathise with your plight with your son! I too am the mum of a newby. First he decided he wanted an axolotl, after much research discovering we needed water chillers etc etc I convinced him fish might be an easier bet!

Ha ha ha , yeah right!!! We already had 2 platy ( mum and baby) having killed off the other 3 and the other baby, in a small 10l ( I think about 6.5G in US) which was a “ starter kit” from a couple of years ago. Having kicked the axolotyl idea in favour of tropical fish, he promptly spent all his hard saved money on a new start up 58l (18/19g) tank, sand, decorations etc etc.followed by 6 fish……4 of which promptly died!! After cycling we were left with 2 black skirt tetra, he then bought 2 lovely small gourami which lasted 2 weeks ( replacing fish will cost around £10/go bless him - about $13)

we are now kind of stuck either throwing more money at it or losing several hundred pounds. The shop replaced the gourami luckily and he got corys instead as we never knew why they died I can only assume they didn’t like the water ie ph/hardness as the nitrites and ammonia were fine. They’re cute and I’m finding the fish rather more interesting and entertaining than I thought and he loves them. He wants to get a betta in the smaller tank which again sounds faffy… so we will just be spending more money; his Christmas list consists of fish related items!

hope you manage to sort your platy.