Betta abdominal swelling - looking for new ideas

[Zebulun]

Looking for new ideas on what the unilateral (right side) abdominal swelling could be on my betta, and what other treatment I could do to resolve the lump.

He's about 3 yrs. old (I've owned him 2 years, bought from PetSmart March 5, 2019, so he is probably 3 yrs old total).  He is alone in a 5 gallon filtered, heated tank with live plants.  pH=7.8, Ammonia=0, Nitrite=0, Nitrate=5 (using API master test kit).  Water temp is 79.  I don't own the chemicals for testing water hardness, but here is a water quality report for my area: http://m.olympiawa.gov/~/media/Files/PublicWorks/Water-Resources/WaterQualityReport.pdf?la=en and water hardness is supposed to rank about 83 ppm according to https://www.hydroflow-usa.com/washington-water-hardness. 
Feb. 3: (see bad photo below with yellow arrow pointing to lump...it's not a good photo) Noticed a swelling on betta's right side/underbelly.  I did 50% water change and added 1 dose API erythromycin, fasted him for 36 hours.
Feb 4: Added second dose API erythromycin.  He made at least 3 large poops.
Feb 5:  He turned deathly pale and developed pop-eye on the ipsilateral side as the abdominal lump. I fed him some frozen brine shrimp in the evening.
Feb 6: Fed him 1/4 cooked pea morning and evening.
Feb 7-11: Continued with Maracyn erythromycin treatment.  The pop-eye healed.  He is still visually impaired.  Fins/body regained their color.
Feb 12: 25% water change.
Feb 15: Did an epsom salt dip; 1 tbsp per gallon for 12 minutes, 2 minutes in a revival bath, then back into his tank.

Food:  Historically, I've fed him mostly Hikari betta bio-gold pellets (probably too much), and occassionaly freeze dried blood worms and freeze-dried brine shrimp.  Added fluval bug bite flakes last month.  Added frozen brine shrimp and cooked peas to his diet in rotation since Feb. 3.  I feed him morning and evening, a portion the size of his eye, rehydrating the pellets and blood worm before feeding.

Behavior:  currently he is active, sociable, hungry, eating well and swimming normally.  The photo below is from Feb. 3 and he looks miserable, but he looks healthier now, I haven't been able to get a clear photo of him yet today.  Will update photo soon.

As of this moment, there has been no change in the lump on his right side.  Neither the erythromycin nor the epsom salt bath have changed the lump.

[FrozenFins]

It doesn't look like anything to worry about. If the dot hasn't grown and the fish is acting fine, I wouldn't do any treatment. It could be a tumor starting at which the only treatment is surgery.

[Zebulun]

The photo is deceiving.  It's more of a lump that's masked by his right fin.  The white spot the arrow is pointing to is just a small spot at the top of the lump.  The lump does not appear to be constipation and was not changed by six days of erythromycin.  So I'm wondering if it's something weird like an abcess, maybe internal parasites (which he could only have gotten from a snail that I got last year and then died), fatty liver, or maybe an internal tumor.  Wondering if it's worth it to try any other treatments, given his age, if additional medicine would be worse that the disease (what it is). 

[Zebulun]

AHA!  He finally sat still long enough in the right position to get a good photo.  Sorry about the algae. What the heck is that?  Fatty liver?  Abcess? Tumor?  Internal parasite? 

[Colu]

It could be a tumor or a cyst looking at pictures 

[Zebulun]

Maybe I'll try another epsom salt dip.  Epsom salt soaks are good for draining abscesses in humans...so, if there is a cyst or abscess in a fish, maybe it might help.  It popped up overnight (on 02/03/21)...which makes me think less of a tumor than a cyst or abcess, seeing as maracyn and fasting did not change the size or shape of the lump.